Scum (5)
I believe the answer is:
dross
'scum' is the definition.
(I know that dross is a type of scum)
This is the entire clue.
(Other definitions for dross that I've seen before include "Waste from molten metal" , "Rubbish, scum" , "Scum on molten metal" , "Scum on molten metals" , "Something of inferior quality" .)