Guilty party (7)
'guilty party' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for culprit that I've seen before include "One perpetrating crime" , "The one to blame, guilty party" , "baddie" , "Perpetrator of a wrongdoing" , "Offender or guilty party" .)
(Other definitions for perpetrator that I've seen before include "Committer (of a crime)" , "One who commits a bad action" , "One carrying out (crime)" , "One who commits (a bad act)" , "One committing (an offence)" .)