Crossword Genius

Dirty froth or foam (4)

Ross

I believe the answer is:

scum

I believe this is a double definition.

'dirty froth' is the first definition.
(I've seen this before)

'foam' is the second definition.
(I know that scum is a foam)

(Other definitions for scum that I've seen before include "Offscourings" , "dirty film" , "Refuse" , "to be seen on top" , "Floating impurities on liquid" .)

I've seen this clue in The Sun.
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