Sharp as a lot of detectives (4)
I believe the answer is:
acid
'sharp' is the definition.
(I've seen this before)
'as a lot of detectives' is the wordplay.
'a lot of' means to remove the last letter.
'detectives' becomes 'cid'.
'as' with its last letter taken off is 'a'.
'a'+'cid'='ACID'
(Other definitions for acid that I've seen before include "Bitter; biting" , "Opposite of alkali; sour" , "Caustic liquid of many kinds" , "Caustic and biting" , "Sour - LSD" .)