Vocal subtlety (4)
I believe the answer is:
tone
'vocal subtlety' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tone that I've seen before include "Quality or character of sound" , "Quality of a musical or vocal sound" , "Colour quality; note" , "Pitch (sound)" , "Quality of sound, voice say" .)