The most common and most likely answer to this crossword clue is the 14 letter word ANTICONVULSANT
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Buddy explains!
The straight part
We know that "fit fighter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fit fighter = anticonvulsant.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.
Crossword Answer definitions
ANTICONVULSANT noun- An agent that prevents, stops, or lessens convulsions.
- (pharmaceutical effect) Acting as an anticonvulsant.