For every favourite traitor or culprit (11)
I believe the answer is:
perpetrator
'culprit' is the definition.
(synonyms)
'for every favourite traitor or' is the wordplay.
'for every' becomes 'per' ('per' means 'for each' or 'for every').
'favourite' becomes 'pet' (synonyms).
'traitor' becomes 'rat' (rat can mean a treacherous person).
'per'+'pet'+'rat'+'or'='PERPETRATOR'
(Other definitions for perpetrator that I've seen before include "Culprit" , "Committer (of a crime)" , "agent" , "One committing (an offence)" , "One committing (a bad deed)" .)