Floating film of muck (4)

I believe the answer is:

scum

(Other definitions for scum that I've seen before include "Floating muck" , "Film on the surface of a liquid" , "Impurities on water surface" , "Waste matter floating on the surface" , "Impurities floating on surface, or despicable people" .)

I've seen this clue in The Guardian.

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