Crossword Genius

Birdbath gunk (4)

Ross

I believe the answer is:

scum

(Other definitions for scum that I've seen before include "Impurities on top of water or worthless people" , "Floating impurities or worthless people" , "The lowest" , "Impurities on surfaces of liquid" , "Impure film" .)

I've seen this clue in the Universal.
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