The most common and most likely answer to this crossword clue is the 7 letter word BATTLER
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More clues leading to the result BATTLER
Buddy explains!

The straight part
We know that "Fighter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fighter = battler.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 BATTLER mean?
BATTLER noun- One who wages battle against an enemy; a soldier; a general.
- An itinerant worker or unemployed person.
- One who who works hard in the face of adversity.
- (Oxford University) A student who is supplied with provisions from the buttery; formerly, one who paid for nothing but what he called for.