Sharp police unit infiltrating revolutionary spies (6)
I believe the answer is:
acidic
'sharp' is the definition.
(I've seen this before)
'police unit infiltrating revolutionary spies' is the wordplay.
'police unit' becomes 'CID' (abbreviation for Criminal Investigation Department).
'infiltrating' means one lot of letters goes inside another.
'revolutionary' says the letters should be written backwards.
'spies' becomes 'CIA' (Central Intelligence Agency).
'cia' reversed gives 'aic'.
'cid' inserted into 'aic' is 'ACIDIC'.
(Other definitions for acidic that I've seen before include "Unpleasant" , "Like vinegar" , "Vinegary" , "scathing?" , "Sharp-tasting, sour" .)