Pond dross (4)
I believe the answer is:
'pond dross' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for scum that I've seen before include "Fruit of the oak" , "Floating muck" , "Floating impurities or bad people" , "Impurities floating on surface, or despicable people" , "Worthless group of people" .)