Joker concealing name is a criminal (8)
I believe the answer is:
gangster
'criminal' is the definition.
(gangster is a kind of criminal)
'joker concealing name' is the wordplay.
'joker' becomes 'gagster' (I've seen this before).
'concealing' means one lot of letters goes inside another.
'name' becomes 'n' (common abbreviation e.g. on forms).
'gagster' going around 'n' is 'GANGSTER'.
'is a' acts as a link.
(Other definitions for gangster that I've seen before include "Organised criminal" , "Mobster" , "Typical Cagney role" , "Hood, desperado" , "Mafia man" .)