Ace police department getting sharp (4)
I believe the answer is:
acid
'sharp' is the definition.
(I've seen this before)
'ace police department getting' is the wordplay.
'ace' becomes 'a'.
'police department' becomes 'cid' (I've seen this in other clues).
'getting' says to put letters next to each other.
'a'+'cid'='ACID'
(Other definitions for acid that I've seen before include "Hallucinogenic drug" , "It is neutralized by alkali" , "Etcher's fluid" , "Tart - keen - sort of house party" , "Substance which turns litmus red" .)