A team of detectives is sharp (4)
I believe the answer is:
acid
'sharp' is the definition.
(as in sharp-tasting)
'a team of detectives' is the wordplay.
'team of detectives' becomes 'CID' (Criminal Investigation Department).
'a'+'cid'='ACID'
'is' acts as a link.
(Other definitions for acid that I've seen before include "It is neutralised by alkali" , "something sharp" , "Bitter; biting" , "Tart, sour" , "Bitter or sour" .)