Lump of metal (4)
I believe the answer is:
slug
'lump of metal' is the definition.
This is the entire clue.
(Other definitions for slug that I've seen before include "Wallop" , "Shell-less mollusc ? bullet" , "Pellet; deal thumping blows" , "Kind of snail without a shell - or a bullet" , "Projectile fired from a gun" .)