Smelting residue (4)
I believe the answer is:
slag
'smelting residue' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for slag that I've seen before include "Waste left after metal is taken from ore" , "Smelting or coal waste" , "Perfect" , "a girl who puts out" , "Cinders could become this" .)