Decorate composition of notes (7)
I believe the answer is:
festoon
'decorate' is the definition.
(I know that festoon is a more specific form of the action decorate)
'composition of notes' is the wordplay.
'composition' indicates an anagram.
'of'+'notes'='ofnotes'
'ofnotes' with letters rearranged gives 'FESTOON'.
(Other definitions for festoon that I've seen before include "Ornamental garland" , "Garland with flowers" , "swag" , "Ornamental chain/garland" , "Decoration" .)