Drug found in a police department (4)
I believe the answer is:
acid
'drug' is the definition.
(acid is a kind of drug)
'a police department' is the wordplay.
'police department' becomes 'cid' (I've seen this in other clues).
'a'+'cid'='ACID'
'found in' is the link.
(Other definitions for acid that I've seen before include "Vinegary" , "Sour - type of rain or drop" , "Caustic liquid of many kinds" , "Morose" , "Corrosive substance" .)