Excuse boxer breeder intended to begin with (5)
I believe the answer is:
alibi
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'boxer breeder intended to begin with' is the wordplay.
'boxer' becomes 'Ali' (Muhammad Ali).
'to begin with' suggests taking the first letters.
The first letters of 'breeder intended' is 'bi'.
'ali'+'bi'='ALIBI'
(Other definitions for alibi that I've seen before include "I bail (anag.)" , "Accused person's defence" , "Cover story" , "Excuse for not doing something" , "Claim put to court" .)