Eton changed in quality (4)
I believe the answer is:
tone
'quality' is the definition.
('tone' can be a synonym of 'quality')
'eton changed' is the wordplay.
'changed' indicates an anagram.
'eton' anagrammed gives 'TONE'.
'in' is the link.
(Other definitions for tone that I've seen before include "Musical note; general character" , "Quality or character of voice" , "Quality of sound, etc" , "Note as in voice" , "Sound signal on a telephone" .)