Policemen heard in thicket (5)
I believe the answer is:
copse
'thicket' is the definition.
(I know that thicket can be written as copse)
'policemen heard' is the wordplay.
'policemen' becomes 'cops' (cop is a kind of policeman).
'heard' indicates a 'sounds like' (homophone) clue.
'cops' sounds like 'COPSE'.
'in' acts as a link.
(Other definitions for copse that I've seen before include "Piece of managed woodland" , "Scope for a small wood" , "Small group of trees" , "Coppice" , "Cluster of trees" .)