Police start to encircle group of trees (5)
I believe the answer is:
copse
'group of trees' is the definition.
(small woodland area)
'police start to encircle' is the wordplay.
'police' becomes 'cops' (I've seen this in another clue).
'start to' suggests taking the first letters.
The first letter of 'encircle' is 'e'.
'cops'+'e'='COPSE'
(Other definitions for copse that I've seen before include "Cultivated woodland area" , "Dense growth of shrubs or trees" , "Stand of trees" , "Clump of small trees for regular cropping" , "Cluster of trees" .)