Riff-raff making unpleasant film (4)
I believe the answer is:
scum
I believe this clue is a double definition.
'riff-raff' is the first definition.
'unpleasant film' is the second definition.
'making' acts as a link.
(Other definitions for scum that I've seen before include "Impurities on surface of liquid, or rotten people" , "Waste matter floating on the surface" , "Floating film of filth" , "Dirt on liquid surface" , "Offscourings" .)