Small change for policeman (6)
I believe the answer is:
copper
I believe this clue is a double definition.
'small change' is the first definition.
(low-value coinage)
'for policeman' is the second definition.
(copper is a kind of policeman)
(Other definitions for copper that I've seen before include "Little money" , "Metal, Cu" , "Reddish metal of a penny" , "Metal for slang policeman" , "Metallic element and British bobby" .)