Melody, not just any note (5)
I believe the answer is:
theme
'melody' is the definition.
(theme is a kind of melody)
'not just any note' is the wordplay.
'not just any' becomes 'the' (I've seen this in another clue).
'note' becomes 'me' (musical note as in do-re-me).
'the'+'me'='THEME'
(Other definitions for theme that I've seen before include "Musical idea; topic" , "(Writer's) topic" , "Pervading idea in art" , "thing under discussion" , "Melody on which variations are constructed" .)