Excuse a bishop interrupting liberal couple (5)
I believe the answer is:
alibi
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'a bishop interrupting liberal couple' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'interrupting' indicates putting letters inside.
'liberal' becomes 'l' (abbreviation).
'couple' becomes 'ii' (I've seen this in other clues).
'l'+'ii'='lii'
'b' going inside 'lii' is 'libi'.
'a'+'libi'='ALIBI'
(Other definitions for alibi that I've seen before include "Defence against charge - was seen elsewhere" , "Excuse for not doing something" , "excuse given" , "Excuse, in law" , "Justification" .)