Foul film, terrible music, I left! (4)
I believe the answer is:
scum
'foul film' is the definition.
(I've seen this in another clue)
'terrible music i left' is the wordplay.
'terrible' indicates an anagram.
'left' is a deletion indicator (some letters have left).
'music' with 'i' removed is 'musc'.
'musc' is an anagram of 'SCUM'.
(Other definitions for scum that I've seen before include "to be seen on top" , "dirty film" , "Dross" , "Dirty froth" , "Impurities on liquid's surface" .)