When I go into an battlefield, then Moonfall an enemy Overzealous Fan into the battlefield, is he still able to use the Overzealous Fan trigger or is the "when I defend" trigger already gone?

Yes, Overzealous Fan can still use its "When I defend" trigger in this scenario.

The Timing Explained

According to the Unleashed rules update (as clarified in RiftJudge FAQ #9611 and FAQ #9174), "When I defend" is a Defend Trigger. These abilities trigger the first time a unit gains the Defender designation during a combat showdown.

If an Overzealous Fan is moved into an ongoing combat (such as via Moonfall), it gains the Defender designation upon entering the battlefield. Because it is gaining that designation for the first time in that combat, its ability triggers at that time, even though the combat was already underway.

Execution Sequence

1. Movement: You play Moonfall and move the opponent’s Overzealous Fan to the contested battlefield.
2. Trigger: Upon entering the battlefield, Overzealous Fan is designated as a Defender. This triggers its "When I defend" ability.
3. Optional Choice: Because the ability text uses "you may," the trigger is optional. The opponent (the controller of the Overzealous Fan) chooses whether or not to place the ability on the chain (Rule 383.3.a).
4. Paying the Cost: Per RiftJudge FAQ #8702 and FAQ #9611, if the opponent chooses to use the ability, they must kill the Overzealous Fan immediately as an upfront cost to place the ability on the chain.
5. Chain Resolution: If the cost was paid, the ability sits on the chain as a pending item. Players may now play [REACTION] cards before the ability resolves. When it resolves, the attacking unit is moved to its base.

Key Summary: The "When I defend" trigger is not "gone" because the Fan entered the battlefield after combat started; the ability triggers specifically because it gains the Defender designation upon arrival. Furthermore, please note that the "kill me" cost is paid upfront by the controller to place the ability on the chain, not upon resolution of the ability itself.

Note: I could not fully verify this ruling against the retrieved rules.