Vitriolic article on a police department (4)
I believe the answer is:
acid
'vitriolic' is the definition.
(synonyms)
'article on a police department' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'a police department' becomes 'CID' (abbreviation for Criminal Investigation Department).
'a'+'cid'='ACID'
(Other definitions for acid that I've seen before include "Tart - keen - sort of house party" , "Caustic liquid of many kinds" , "It is neutralised by alkali" , "Sour, sharp-tasting" , "something sharp" .)