Bad cop's diet is oppressive (8)
I believe the answer is:
despotic
'oppressive' is the definition.
(thesaurus)
'bad cop's diet' is the wordplay.
'bad' indicates an anagram.
'cops'+'diet'='copsdiet'
'copsdiet' with letters rearranged gives 'DESPOTIC'.
'is' acts as a link.
(Other definitions for despotic that I've seen before include "Like the Czar" , "Cruelly oppressive" , "Referring to absolute power" , "Like a cruel ruler" , "Authoritarian" .)