Defence of boxer protecting middle of ribs (5)
I believe the answer is:
alibi
'defence' is the definition.
(defence given to police by a suspect)
'boxer protecting middle of ribs' is the wordplay.
'boxer' becomes 'Ali' (Muhammad Ali).
'protecting' means one lot of letters goes inside another (inserted letters are held or protected).
'middle of' says to take the centre.
The centre of 'ribs' is 'ib'.
'ali' enclosing 'ib' is 'ALIBI'.
'of' acts as a link.
(Other definitions for alibi that I've seen before include "Timely excuse" , "I bail (anag.)" , "Justification" , "Evidence one was not present" , "Proof of not being at crime site" .)