Note changes in quality (4)
I believe the answer is:
tone
'quality' is the definition.
('tone' can be a synonym of 'quality')
'note changes' is the wordplay.
'changes' is an anagram indicator.
'note' anagrammed gives 'TONE'.
'in' acts as a link.
(Other definitions for tone that I've seen before include "Pitch of a voice" , "Pitch or timbre" , "Theobald Wolf . . . ." , "Shade" , "Quality or character of sound" .)