Bimbo regularly interrupts boxer's defence (5)
I believe the answer is:
alibi
'defence' is the definition.
(alibi is a kind of defence)
'bimbo regularly interrupts boxer's' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'interrupts' indicates putting letters inside.
'boxer' becomes 'ali' (Muhammad Ali).
The alternating letters of 'bimbo' are 'ib'.
'ib' put into 'ali' is 'ALIBI'.
(Other definitions for alibi that I've seen before include "Evidence of (or excuse for) being somewhere else at the time" , "Excuse, defence" , "Legal defence - was elsewhere" , "Claim put to court" , "Story confirming innocence" .)