Dirty froth or foam (4)
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" .)