Music, hackneyed stuff that's supposed to enhance a film (7)
I believe the answer is:
popcorn
'that's supposed to enhance a film' is the definition.
(snack often consumed at the cinema )
'music hackneyed stuff' is the wordplay.
'music' becomes 'pop' (pop music is a kind of music).
'hackneyed stuff' becomes 'corn' (as in cliched or corny material).
'pop'+'corn'='POPCORN'
(Other definitions for popcorn that I've seen before include "Popular snack in the cinema" , "Stuff to eat at the cinema" , "Kernels that swell and burst with heat" , "Puffed maize" , "Filmgoers' diet" .)