Waste matter (5)
I believe the answer is:
dross
'waste matter' is the definition.
(I know that dross is a type of waste matter)
This is the entire clue.
(Other definitions for dross that I've seen before include "Scum - dregs" , "Waste coal - rubbish" , "Something of inferior quality" , "Lees" , "Rubbish, scum" .)