The most common and most likely answer to this crossword clue is the 8 letter word FOREPAWS
I've seen this crossword clue in The Sunday Telegraph
F
O
R
E
P
A
W
S
More clues leading to the result FOREPAWS
Buddy explains!

The straight part
We know that "Walk-on parts for boxer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case walk-on parts for boxer = forepaws.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 FOREPAWS mean?
FOREPAWS noun- Either of the paws of an animal's foreleg, homologous to the hand in humans.