Smelting byproduct
I believe the answer is:
slag
'smelting byproduct' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for slag that I've seen before include "a girl who puts out" , "Mound of waste matter from mine" , "Smelted-ore refuse" , "Solid waste matter as from mine or smelter" , "Coal mining waste" .)