Excuse Mali bigamist uses (5)
I believe the answer is:
alibi
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'mali bigamist uses' is the wordplay.
'uses' indicates a hidden word (the answer is used as part of the clue).
'ALIBI' can be found hidden inside 'mali bigamist'.
(Other definitions for alibi that I've seen before include "Courtroom evidence" , "Defence in court" , "Legal defence, plea" , "Proof of being elsewhere at the time of a crime." , "can offer a valid defence against most charges" .)