Tone quality (6)
I believe the answer is:
timbre
'tone quality' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for timbre that I've seen before include "Quality of voice, sounds wooden" , "Quality of sound of the voice" , "Tone of voice - sounds wooden" , "Distinctive tone" , "The distinctive property of a voice" .)