Dross (4)
I believe the answer is:
scum
'dross' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for scum that I've seen before include "Impurities that rise to surface of liquid" , "People regarded as worthless, especially criminals" , "Floating dirt on surface" , "Waste matter floating on the surface" , "Floating rubbish" .)