The most common and most likely answer to this crossword clue is the 12 letter word WELTERWEIGHT
I've seen this crossword clue in The Guardian
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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 = welterweight.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 WELTERWEIGHT mean?
WELTERWEIGHT noun- A boxer weighing more than a lightweight boxer and less than a middleweight boxer; someone boxing in the welterweight class
- A weight of 28 pounds (or 40 pounds: a heavy welterweight), sometimes imposed in addition to weight for age, chiefly in steeplechases and hurdle races.
- Between lightweight and middleweight