Cast initially appearing in whole film (4)
I believe the answer is:
scum
'film' is the definition.
(scum is a kind of film)
'cast initially appearing in whole' is the wordplay.
'initially' suggests taking the first letters.
'appearing in' indicates putting letters inside.
'whole' becomes 'sum' (sum is a kind of whole).
The first letter of 'cast' is 'c'.
'c' inserted inside 'sum' is 'SCUM'.
(Other definitions for scum that I've seen before include "Floating dirt or froth" , "Fruit of the oak" , "Algae" , "low life" , "Impurities floating on water, or despicable people" .)