Combination of melodies from nocturne I opt to change (12)
I believe the answer is:
'combination of melodies' is the definition.
(I've seen this before)
'nocturne i opt to change' is the wordplay.
'to change' indicates anagramming the letters.
'nocturneiopt' with letters rearranged gives 'COUNTERPOINT'.
'from' is the link.
(Other definitions for counterpoint that I've seen before include "Melody added to another" , "Combination of melodies" , "Interplay of melodies" , "Foil" , "Compensate for" .)