Sharp as a group of police (4)
I believe the answer is:
acid
'sharp' is the definition.
(as in sharp-tasting)
'a group of police' is the wordplay.
'group of police' becomes 'CID' (abbreviation for Criminal Investigation Department).
'a'+'cid'='ACID'
'as' is the link.
(Other definitions for acid that I've seen before include "Bitter substance" , "So-called drug" , "See 4 Across" , "- - -drop, sweet" , "Bitter; biting" .)