A police department gets psychedelic drug (4)
I believe the answer is:
acid
'psychedelic drug' is the definition.
(acid is a kind of psychedelic drug)
'a police department' is the wordplay.
'police department' becomes 'cid' (abbreviation for Criminal Investigation Department).
'a'+'cid'='ACID'
'gets' acts as a link.
(Other definitions for acid that I've seen before include "Sour - sharp" , "Substance with pH of less than 7" , "So-called drug" , "- - -drop, sweet" , "Sour tasting" .)