Unpleasant film on the surface (4)

I believe the answer is:

scum

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'unpleasant film' is the definition.
(as in a film of scum on top of a liquid)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for scum that I've seen before include "Impurities on liquid's surface" , "Impurities on surface of liquid - or bad people" , "Impurities floating on surface, or despicable people" , "Dirt on liquid surface" , "Impurities floating on water, or despicable people" .)

I've seen this clue in The Irish News.

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