Drink up - that should stop a policeman coming back (5)
I believe the answer is:
cuppa
'drink' is the definition.
(cuppa is a kind of drink)
'up that should stop a policeman coming back' is the wordplay.
'that should stop' means one lot of letters goes inside another (stopping can mean blocking a gap).
'policeman' becomes 'PC' (police constable).
'coming back' shows that the letters should be reversed in order.
'a'+'pc'='apc'
'apc' backwards is 'cpa'.
'up' placed inside 'cpa' is 'CUPPA'.
(Other definitions for cuppa that I've seen before include "Drink of tea (informal)" , "Drink of tea (colloq.)" , "Refreshing brew" .)