Layer on a stagnant pond (4)
I believe the answer is:
scum
'layer on a stagnant pond' 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 "People regarded as worthless, especially criminals" , "Impurities on surfaces of liquid" , "Impurities on surface of liquid, or rotten people" , "Dirty foam" , "Impure film" .)