Crossword Genius

Bathtub buildup (4)

Ross

I believe the answer is:

scum

'bathtub buildup' 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 "Dregs (of society?)" , "Worthless group of people" , "Impurities on surface of liquid, or rotten people" , "lowlife" , "Refuse" .)

I've seen this clue in the LA Times and The Wall Street Journal.
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