Flatten metal (4)
I believe the answer is:
iron
I believe this clue is a double definition.
'flatten' is the first definition.
(I know this)
'metal' is the second definition.
(iron is a metal)
(Other definitions for iron that I've seen before include "Implacable - golf club" , "Strong metal; golf club" , "sort of club" , "Heavy metal that rusts easily" , "Main part of steel" .)