Bull kept by oppressive dictator (5)
I believe the answer is:
edict
'bull' is the definition.
(as in a papal bull, an edict from the pope)
'kept by oppressive dictator' is the wordplay.
'kept by' says the answer is hidden in the clue.
'EDICT' is hidden within 'oppressive dictator'.
(Other definitions for edict that I've seen before include "Official decree" , "Injunction" , "Legally binding proclamation" , "Decree, order" , "Cited an official proclamation" .)