Secret police try to conceal doctored tapes (7)
I believe the answer is:
gestapo
'secret police' is the definition.
(Nazi secret police)
'try to conceal doctored tapes' is the wordplay.
'try' becomes 'go' (both can mean an attempt).
'to conceal' is an insertion indicator.
'doctored' indicates anagramming the letters.
'tapes' anagrammed gives 'estap'.
'go' going around 'estap' is 'GESTAPO'.
(Other definitions for gestapo that I've seen before include "Postage to Nazi secret police" , "Brutal force" , "Notorious secret police" , "Postage (anag)" , "Postage for Hitler's police" .)