Coal furnace waste (4)
I believe the answer is:
slag
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">coal 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 "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" .)