The most common and most likely answer to this crossword clue is the 4 letter word CLAY
I've seen this crossword clue in The Times
C
L
A
Y
More clues leading to the result CLAY
Buddy explains!

The straight part
We know that "Boxer, once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case boxer, once = clay.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 CLAY mean?
CLAY noun- A mineral substance made up of small crystals of silica and alumina, that is ductile when moist; the material of pre-fired ceramics.
- An earth material with ductile qualities.
- A tennis court surface made of crushed stone, brick, shale, or other unbound mineral aggregate.
- The material of the human body.
- A particle less than 3.9 microns in diameter, following the Wentworth scale.
- A clay pipe for smoking tobacco.
- A clay pigeon.
- Land or territory of a country or other political region, especially when subject to territorial claims
- To add clay to, to spread clay onto.
- (of sugar) To purify using clay.