Combination of melodies from nocturne I opt to change (12)
I believe the answer is:
counterpoint
'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.
'nocturne'+'i'+'opt'='nocturneiopt'
'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" .)