The most common and most likely answer to this crossword clue is the 7 letter word BRUISER
I've seen this crossword clue in The Times
B
R
U
I
S
E
R
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 = bruiser.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 BRUISER mean?
BRUISER noun- In contact sports, an athlete whose size, strength, and/or aggressiveness make it likely that he will cause athletes on the opposing team to suffer physical punishment.
- (by extension) a tall, strong, heavily built man, especially one prone to physical violence; a thug.
- A machine for bruising oats.