Drug discovered by a police department (4)
I believe the answer is:
acid
'drug' is the definition.
(acid is a kind of drug)
'by a police department' is the wordplay.
'by' says to put letters next to each other.
'police department' becomes 'CID' (abbreviation for Criminal Investigation Department).
'a'+'cid' is 'ACID'.
'discovered' is the link.
(Other definitions for acid that I've seen before include "Bitter - drug" , "Tart - keen - sort of house party" , "bad rain" , "Chemical compound" , "scathing" .)