Drug from 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'
'from' acts as a link.
(Other definitions for acid that I've seen before include "Substance with pH of less than 7" , "Substance which turns litmus red" , "scathing" , "Bitter or sour" , "Chemical compound" .)