Bishop, one supporting boxer’s defence (5)
I believe the answer is:
alibi
'defence' is the definition.
(defence given to police by a suspect)
'bishop one supporting boxer's' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'one' becomes 'I' (Roman numeral).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'boxer' becomes 'ali' (Muhammad Ali).
'b'+'i'='bi'
'bi' put after 'ali' is 'ALIBI'.
(Other definitions for alibi that I've seen before include "Excuse, in law" , ""suspect's story"" , "Making excuse" , "Accused person's defence" , "Proof of being somewhere else at the time" .)