Criminal is bad in vain (7)
I believe the answer is:
villain
'criminal' is the definition.
(thesaurus)
'bad in vain' is the wordplay.
'bad' becomes 'ill' ('ill' can be similar in meaning to 'bad').
'in' means one lot of letters goes inside another.
'ill' put inside 'vain' is 'VILLAIN'.
'is' acts as a link.
(Other definitions for villain that I've seen before include "Criminal, rogue" , "Principal fictional baddie" , "Bad person in story/play" , "Bad guy" , "Criminal; stage baddy" .)