Sweet police found in a ditch (4,4)
I believe the answer is:
acid drop
'sweet' is the definition.
(I've seen this before)
'police found in a ditch' is the wordplay.
'police' becomes 'CID' (abbreviation for Criminal Investigation Department).
'found in' indicates putting letters inside.
'ditch' becomes 'drop'.
'a'+'drop'='adrop'
'cid' going into 'adrop' is 'ACID DROP'.
(Other definitions for acid drop that I've seen before include "Boiled sweet with sharp taste" , "Sharp tasting boiled sweet" , "Lemony candy" .)