Boxer biter (4)
I believe the answer is:
flea
'boxer biter' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for flea that I've seen before include "In your ear, this insect is a rebuke" , "Leaping insect" , "Jumping, biting insect" , "Biting type" , "In one's ear, it's a stinging rebuke" .)