What a dirty film! (4)
I believe the answer is:
scum
'what a dirty film' 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 "The lowest" , "Floating rubbish" , "Waste matter floating on the surface" , "Impurities on surface of liquid - or bad people" , "worst kind of people" .)