Quality of tone (6)
I believe the answer is:
timbre
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">quality of tone</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for timbre that I've seen before include "Distinctive sound quality" , "colour we hear" , "Tone of voice - sounds wooden" , "Quality of musical sound" , "Musical tone quality" .)