Excuse boxer over "half bite" (5)
I believe the answer is:
alibi
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'boxer over half bite' is the wordplay.
'boxer' becomes 'ali' (Muhammad Ali).
'over' means one lot of letters go next to another.
'half' indicates taking half.
'bite' with half the letters taken is 'bi'.
'ali'+'bi'='ALIBI'
(Other definitions for alibi that I've seen before include "Defence against charge - was seen elsewhere" , "Claim put to court" , "Excuse - plea of being elsewhere" , "Courtroom evidence" , "Defence against accusation - was seen elsewhere at the time" .)