Despicable people in dirty film? (4)
I believe the answer is:
scum
This clue is a double definition.
'despicable people' is the first definition.
(I've seen this before)
'dirty film?' is the second definition.
(I've seen this before)
'in' acts as a link.
(Other definitions for scum that I've seen before include "Worthless man" , "Refuse" , "Impurities that rise to surface of liquid" , "Undesirable people" , "Matter floating on 9 ac" .)