Joker ensnares Batman finally? That's criminal! (8)
I believe the answer is:
gangster
'criminal' is the definition.
(gangster is a kind of criminal)
'joker ensnares batman finally?' is the wordplay.
'joker' becomes 'gagster' (I've seen this before).
'ensnares' means one lot of letters goes inside another.
'finally?' indicates one should take the final letters (I've seen 'finally' mean this).
The final letter of 'batman' is 'n'.
'gagster' enclosing 'n' is 'GANGSTER'.
'that's' is the link.
(Other definitions for gangster that I've seen before include "Hood, desperado" , "Wise guy" , "Soprano possibly" , "Villain" , "Member of a mob like Al Capone" .)