Pond film (4)
I believe the answer is:
scum
'pond film' 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 "Floating film of filth" , "Impurities on top of water or worthless people" , "Impure film" , "Matter floating on 9 ac" , "Film on the surface of a liquid" .)