The most common and most likely answer to this crossword clue is the 12 letter word BANTAMWEIGHT
I've seen this crossword clue in The Daily Telegraph
B
A
N
T
A
M
W
E
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Buddy explains!
The straight part
We know that "How one might categorise fighter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case how one might categorise fighter = bantamweight.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 BANTAMWEIGHT mean?
BANTAMWEIGHT noun- A weight class in boxing and other sports, intermediate between flyweight and featherweight. In boxing it ranges from 112 to 118 pounds (51 to 54 kg).
- A boxer or other competitor of this weight.