Sharp answer by detectives (4)
I believe the answer is:
acid
'sharp' is the definition.
(I've seen this before)
'answer by detectives' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'by' says to put letters next to each other.
'detectives' becomes 'cid'.
'a'+'cid'='ACID'
(Other definitions for acid that I've seen before include "something sharp" , "Type of chemical" , "Drug, tart" , "It turns litmus paper red" , "Opposite of alkali; sour" .)