Steel mill residue (4)
I believe the answer is:
slag
'steel mill residue' 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 "Scum on metal - dissolute woman" , "Waste left over from coal-mining" , "Waste left after metal is taken from ore" , "Mound of waste matter from mine" , "Cinder" .)