Those one despises in unpleasant film (4)
I believe the answer is:
scum
This clue is a double definition.
'those one despises' is the first definition.
(you might call people you despise 'scum')
'unpleasant film' is the second definition.
(as in a film of scum on top of a liquid)
'in' acts as a link.
(Other definitions for scum that I've seen before include "Dirt or froth on a liquid" , "Impurities floating on water, or despicable people" , "Floating film of filth" , "Impurities on surface of liquid - or bad people" , "Floating dirt or froth" .)