Famous boxer, British, I excuse (5)
I believe the answer is:
alibi
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'famous boxer british i' is the wordplay.
'famous boxer' becomes 'ali' (I've seen this in another clue).
'british' becomes 'b' (abbreviation e.g. in BBC).
'ali'+'b'+'i'='ALIBI'
(Other definitions for alibi that I've seen before include "Proof of not being at the scene of the crime" , "Legal proof of innocence" , "Fact of being elsewhere (legal)" , "Lawyer's weapon, Latin for 'elsewhere'" , "Timely excuse" .)