Conceited about sick lawbreaker (7)
I believe the answer is:
villain
'lawbreaker' is the definition.
(thesaurus)
'conceited about sick' is the wordplay.
'conceited' becomes 'vain' ('vain' can be a synonym of 'conceited').
'about' is an insertion indicator.
'sick' becomes 'ill' ('ill' can be a synonym of 'sick').
'vain' enclosing 'ill' is 'VILLAIN'.
(Other definitions for villain that I've seen before include "Criminal, rogue" , "Bad person in story/play" , "Evil character" , "Principal fictional baddie" , "Bad guy" .)