Improvise most of alibi about diamonds (2-3)
I believe the answer is:
ad-lib
'improvise' is the definition.
(I've seen this before)
'most of alibi about diamonds' is the wordplay.
'most of' means to remove the last letter.
'about' indicates putting letters inside.
'diamonds' becomes 'd' (abbreviation for the card suit diamonds).
'alibi' with its final letter taken off is 'alib'.
'alib' enclosing 'd' is 'AD-LIB'.
(Other definitions for ad-lib that I've seen before include "Improvise" , "spontaneous comment" , "Unprepared remark" , "Speak impromptu" , "freely" .)