Refinery refuse (4)
I believe the answer is:
slag
'refinery refuse' 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" , "Smelting or coal waste" , "Waste from ore refining" , "Waste left over from coal-mining" , "Perfect" .)