Firm understood to be sacking leader colluding in crime (9)
I believe the answer is:
complicit
'colluding in crime' is the definition.
(complicit in criminal activity)
'firm understood to be sacking leader' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'understood' becomes 'implicit' (similar in meaning).
'to be sacking leader' means to remove the first letter (the first letter is removed or sacked).
'implicit' with its first letter taken off is 'mplicit'.
'co'+'mplicit'='COMPLICIT'
(Other definitions for complicit that I've seen before include "Involved with others in law-breaking" , "Being party to a crime" , "not without guilt?" , "Involved with others in illegality" , "Conniving" .)