Coal furnace waste (4)
I believe the answer is:
slag
'coal 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 "Waste left after metal is taken from ore" , "Waste left over from coal-mining" , "dissolute man (more recently)" , "Waste from ore refining" , "Cinders could become this" .)