The most common and most likely answer to this crossword clue is the 3 letter word FAN
I've seen this crossword clue in The Guardian
F
A
N
More clues leading to the result FAN
Buddy explains!

The straight part
We know that "Zealot" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case zealot = fan.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does FAN mean?
FAN noun- A hand-held device consisting of concertinaed material, or slats of material, gathered together at one end, that may be opened out into the shape of a sector of a circle and waved back and forth in order to move air towards oneself and cool oneself.
- An electrical or mechanical device for moving air, used for cooling people, machinery, etc.
- The action of fanning; agitation of the air.
- Anything resembling a hand-held fan in shape, e.g., a peacock’s tail.
- An instrument for winnowing grain, by moving which the grain is tossed and agitated, and the chaff is separated and blown away.
- A small vane or sail, used to keep the large sails of a smock mill always in the direction of the wind.
- A section of a tree having a finite number of branches
- To blow air on (something) by means of a fan (hand-held, mechanical or electrical) or otherwise.
- To slap (a behind, especially).
- (usually to fan out) To move or spread in multiple directions from one point, in the shape of a hand-held fan.
- To dispel by waving a hand-held fan.
- To perform a maneuver that involves flicking the top rear of an old-style gun.
- (metaphoric) To invigorate, like flames when fanned.
- To winnow grain.
- A person who is fond of something or someone, especially an admirer of a performer or aficionado of a sport.