Part of film score, perhaps minus canticle I'd composed (10,5)
I believe the answer is:
incidental music
'part of film score perhaps' is the definition.
(incidental music plays during scenes)
'minus canticle i'd composed' is the wordplay.
'composed' is an anagram indicator.
'minus'+'canticle'+'id'='minuscanticleid'
'minuscanticleid' anagrammed gives 'INCIDENTAL MUSIC'.
(Other definitions for incidental music that I've seen before include "Notes on film" , "Background to a film or play" , "that creates a mood" .)