Dirty froth (4)
I believe the answer is:
scum
'dirty froth' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for scum that I've seen before include "Offscourings" , "Floating dirt or froth" , "Impurities on surface of liquid - or bad people" , "Impurities floating on surface, or despicable people" , "Floating film of filth" .)