Sharp as a group of detectives (4)
I believe the answer is:
acid
'sharp' is the definition.
(as in sharp-tasting)
'a group of detectives' is the wordplay.
'group of detectives' becomes 'CID' (Criminal Investigation Department).
'a'+'cid'='ACID'
'as' is the link.
(Other definitions for acid that I've seen before include "Sharp - tart - kind of house party" , "Caustic liquid of many kinds" , "something sharp" , "Tart - keen - sort of house party" , "Corrosive liquid; drug" .)