American lawyer turns liberal, quite off the cuff (2, 3)
I believe the answer is:
ad lib
'quite off the cuff' is the definition.
'american lawyer turns liberal' is the wordplay.
'american lawyer' becomes 'da' (abbreviation for District Attorney).
'turns' is a reversal indicator.
'liberal' becomes 'lib'.
'da' back-to-front is 'ad'.
'ad'+'lib'='AD-LIB'
(Other definitions for ad lib that I've seen before include "Vamp" , "with no forethought" , "Speak unprepared" , "Freely available" , "Leave script" .)