The most common and most likely answer to this crossword clue is the 13 letter word FEATHERWEIGHT
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More clues leading to the result FEATHERWEIGHT
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 = featherweight.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 FEATHERWEIGHT mean?
FEATHERWEIGHT noun- A weight class in many combat sports; e.g. in professional boxing of a maximum of 126 pounds or 57.2 kilograms.
- A sportsman who fights in this division.
- The lightest weight that may be carried by a racehorse.