Acerbic article on a police department (4)
I believe the answer is:
acid
'acerbic' is the definition.
('acid' can be a synonym of 'acerbic')
'article on a police department' is the wordplay.
'article' becomes 'a' (indefinite article).
'on' is a charade indicator (letters next to each other).
'a police department' becomes 'cid' (I've seen this in other clues).
'a'+'cid'='ACID'
(Other definitions for acid that I've seen before include "Sour, sharp-tasting" , "Tart, sour" , "Tart - keen - sort of house party" , "Sour - type of rain or drop" , "Caustic liquid of many kinds" .)