Waste matter floating on the surface (4)
I believe the answer is:
scum
'waste matter floating on the surface' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for scum that I've seen before include "The lowest" , "Floating dirt or froth" , "Dirty foam" , "Impurities on liquid's surface" , "rabble" .)