Furnace waste (4)
I believe the answer is:
slag
'furnace waste' 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 "Scum on metal - dissolute woman" , "Waste from ore refining" , "a girl who puts out" , "Smelted-ore refuse" , "Mound of waste matter from mine" .)