Policemen trailing a tart (4)
I believe the answer is:
acid
'tart' is the definition.
(thesaurus)
'policemen trailing a' is the wordplay.
'policemen' becomes 'cid' (I've seen this in other clues).
'trailing' is a charade indicator (letters next to each other).
'cid' put after 'a' is 'ACID'.
(Other definitions for acid that I've seen before include "Corrosive substance" , "Corrosive fluid" , "Sharp - tart - kind of house party" , "Bitter - drug" , "bad rain" .)