Group of policemen surrounding area to the west (4)
I believe the answer is:
team
'group' is the definition.
(group of people)
'policemen surrounding area to the west' is the wordplay.
'policemen' becomes 'Met' (Metropolitan Police).
'surrounding' indicates putting letters inside.
'area' becomes 'a' (maths abbreviation).
'to the west' shows that the letters should be reversed in order (letters written right to left or 'east to west').
'met' enclosing 'a' is 'maet'.
'maet' in reverse letter order is 'TEAM'.
'of' acts as a link.
(Other definitions for team that I've seen before include "One side in a game" , "A cooperative unit" , "(Football) side" , "People working together" , "(Sports) side" .)