Secret police fiddling postage (7)
I believe the answer is:
gestapo
'secret police' is the definition.
(Nazi secret police)
'fiddling postage' is the wordplay.
'fiddling' indicates anagramming the letters.
'postage' with letters rearranged gives 'GESTAPO'.
(Other definitions for gestapo that I've seen before include "Postage (anag)" , "Notorious secret police" , "war force" , "Police detested" , "Men in black" .)