Film about police department overturning decree (5)
I believe the answer is:
edict
'decree' is the definition.
('edict' can be a synonym of 'decree')
'film about police department overturning' is the wordplay.
'film' becomes 'et'.
'about' indicates putting letters inside.
'police department' becomes 'cid' (I've seen this in other clues).
'overturning' shows that the letters should be reversed in order.
'cid' in reverse letter order is 'dic'.
'et' going around 'dic' is 'EDICT'.
(Other definitions for edict that I've seen before include "Official order" , "Decree by authority" , "Order issued by an authority" , "Authoritative decree" , "Official decree" .)