Are there any Trek episodes where they enforce the Prime Directive to protect a developing world within Federation space from an external influence?

In Star Trek: Insurrection, the Enterprise protected the Baku from the So'na, though if I remember right, there was some debate as to whether the prime directive applied as the Baku weren't native to the planet.

there was some debate as to whether the prime directive applied as the Baku weren’t native to the planet.
...which is an absolutely insane debate. No one tries to argue that the PD doesn't apply to the Romulans, even though they're not native to their world.
Colony planets aren't considered "fair game" for interference.
- Romulans: Don’t ~~Tread~~ Mintaka On me
And... Spoiler... It was an internal conflict anyway

The Prime Directive is that the Federation won’t interfere with the development of a pre-warp society.

It is NOT foreign policy that they try to enforce against other warp-capable civilizations.

A couple that come to mind is VOY “False Prophets” where they expel a couple parasitic Ferengi or ENT “Civilization” where warp-capable aliens are causing toxic pollution in a pre-warp society.

ENT “Civilization” where warp-capable aliens are causing toxic pollution in a pre-warp society.
I'm going to turbo nitpick here. That episode took place before the Prime Directive was written, so it can't be held up as an example of how the PD is treated.
It is NOT foreign policy that they try to enforce against other warp-capable civilizations.
But this is not about foreign policy! This is about developing planets within Federation space. Federation space is a specific region of the Alpha and Beta quadrants and I'm assuming may refer to both members and non-member worlds within that region.
The only reason I ask is because the Fan page for the Federation says they enforce the prime directive on any invaders in fed space. I think False Profits is probably the only example that I can think of! Enterprise is pre-federation and pre-prime directive unfortunately.
The Prime Directive is that the Federation won’t interfere with the development of a pre-warp society.
While warp capability (or a rough equivalent) is the prerequisite for first contact, the Prime Directive is broader, forbidding intervention in the internal affairs of any non-Federation civilization.
In false prophets, were they in federation space? In civilization did the federation exist?
- No they weren't. Janeway really stretched the Prime Directive to fit what she wanted to do.
  She reasoned that the ferengi were stuck in the delta quandrant during negotiations facilited by the Federation, so therefore the Federation had caused the cultural contamination, so therefore going down to the planet to clean it up was actually following the Prime Directive.

I always felt that they only mentioned the Prime Directive when they were going to violate it

@harrysbalzac @TimewornTraveler They had to have the temptation or it would only be a concept.

SNW S1E2 - Children of the Comet. This is a tricky one because of potential predestination and the end result being an accidental major break of the prime directive. A warp capable civilization is certain that a comet must destroy a pre warp civilization and refuse to let that change. The issue is that the ship/comet wants the Federation to interfere and does not intend to hit the planet. This is all in Federation space.

Another in Federation space that is also a loose fit is LD S5E7 - Fully Dialated where the crew must recover Data's head from an alternate reality that has fallen on a pre warp world. The loose fit here is that the only reason they count as external is that it is Purple Data who is not technically a a member of this realities Federation and thus an outsider.

Lastly a decent fit but outside Federation space is Prodigy S1E7 - First Con-Tact. The crew first breaks the prime directive by making first contact with a pre warp civilization but then defends it by stopping a Ferengi from influence the civilization negatively. This is outside Federation space though.

There was that one episode where they tried to save a planet from Satan. I dont think the planet was technically pre-warp, though, especially since there were federation scientists/engineers on the planet working with the population.

which series
- TNG, Devil's Due
The planet had previously industrialized and since de-industrialized by choice.

There's the TNG episode where Federation scientists are observing a non-warp capable civ from on the planet, when an accident exposed their lab and the aliens take one of the scientists hostage thinking they are demons. In this case, the external influence they are trying to limit and fix is the federation itself.

There is also another in the same vein where Data is damaged during an away mission on a planet with a non-warp capable civ and inadvertently is spreading radioactive material around the settlement because he lost his memories, and then has to correct all the damage when he is repaired.

These are the only two I can think of that even come close while also being within Federation space. As others have pointed out, the Voyager episode where they run into the Farenghi that ended up there from another episode of TNG is the exact thing you are asking... Just not in the right part of space.