Former boxing champ with book — one providing defence (5)
I believe the answer is:
alibi
'defence' is the definition.
(defence given to police by a suspect)
'former boxing champ with book one' is the wordplay.
'former boxing champ' becomes 'ali' (I've seen this in another clue).
'with' says to put letters next to each other.
'book' becomes 'b' (abbreviation for book).
'one' becomes 'i' (Roman numeral).
'ali'+'b'+'i'='ALIBI'
'providing' is the link.
(Other definitions for alibi that I've seen before include "Timely excuse" , "Legal proof of innocence" , "Fact of being elsewhere (legal)" , "Evidence that the accused could not have been guilty" , "Sort of explanation" .)