Eton varies in quality (4)
I believe the answer is:
tone
'quality' is the definition.
(I know that quality can be written as tone)
'eton varies' is the wordplay.
'varies' indicates an anagram.
'eton' is an anagram of 'TONE'.
'in' is the link.
(Other definitions for tone that I've seen before include "Vocal sound of note" , "Pitch (sound)" , "Theobald Wolf . . . ." , "Quality or character of sound" , "Note; note (anag.)" .)