A favourite informant or criminal (11)
I believe the answer is:
perpetrator
'criminal' is the definition.
(I know that criminal can be written as perpetrator)
'a favourite informant or' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'favourite' becomes 'pet' (synonyms).
'informant' becomes 'rat' (rat is a kind of informant).
'per'+'pet'+'rat'+'or'='PERPETRATOR'
(Other definitions for perpetrator that I've seen before include "Committer (of a crime)" , "One committing (a bad deed)" , "responsible individual" , "agent" , "One committing (an offence)" .)