The most common and most likely answer to this crossword clue is the 9 letter word FLYWEIGHT
I've seen this crossword clue in The Times
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More clues leading to the result FLYWEIGHT
Buddy explains!

The straight part
We know that "boxer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case boxer = flyweight.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 FLYWEIGHT mean?
FLYWEIGHT noun- A weight that moves outward depending on centrifugal force.
- A weight class in many combat sports; e.g. in professional boxing of a maximum of 112 pounds or 50.8 kilograms.
- (adjectival use) Small, light or unimportant.