It's a waste
I believe the answer is:
slag
'it's a waste' 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 "Perfect" , "Scum on metal - dissolute woman" , "Smelting or coal waste" , "Mound of waste matter from mine" , "Smelted-ore refuse" .)