Smooth metal (4)
I believe the answer is:
iron
I believe this clue is a double definition.
'smooth' is the first definition.
(to smooth down with an iron)
'metal' is the second definition.
(iron is a metal)
(Other definitions for iron that I've seen before include "Smooth - driver?" , "Wellington was known as the .... Duke" , "Heavy metallic element" , "rations in emergency?" , "Smooth - metal" .)