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' (synonyms).
'about' indicates putting letters inside.
'sick' becomes 'ill' (similar in meaning).
'vain' going around '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" .)