Furnace waste (4)
I believe the answer is:
slag
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">furnace waste</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)