Policeman starts to search extremely small wood (5)
I believe the answer is:
copse
'small wood' is the definition.
(small woodland area)
'policeman starts to search extremely' is the wordplay.
'policeman' becomes 'cop' (slang term).
'starts to' suggests taking the first letters.
The first letters of 'search extremely' is 'se'.
'cop'+'se'='COPSE'
(Other definitions for copse that I've seen before include "Small thicket" , "Small group of trees" , "Coppice" , "Piece of woodland used for cutting" , "Dense growth of bushes or trees" .)