Metal policeman (6)
I believe the answer is:
copper
I believe this is a double definition.
'metal' is the first definition.
(I know that copper is a type of metal)
'policeman' is the second definition.
(copper can informally mean a police officer)
(Other definitions for copper that I've seen before include "peeler" , "One who nicks" , "Reddish-brown metallic element" , "Metallic element and British bobby" , "Atomic number 29" .)