Theme of title I'm adapting (9)
I believe the answer is:
leitmotif
'theme' is the definition.
(a leitmotif is a recurring theme in a work)
'of title i'm adapting' is the wordplay.
'adapting' indicates anagramming the letters.
'of'+'title'+'im'='oftitleim'
'oftitleim' with letters rearranged gives 'LEITMOTIF'.
(Other definitions for leitmotif that I've seen before include "Recurring theme in literary composition" , "it's a repetitive strain" , "Recurrent theme" , "A melodic phrase that accompanies the reappearance of a person on stage" , "Recurring musical theme" .)