Viola's voice (4)
I believe the answer is:
alto
This clue could be a double definition.
'viola's' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'voice' is the second definition.
(alto is a kind of voice)
Can you help me to learn more?
(Other definitions for alto that I've seen before include "The highest adult male singing voice" , "Member of choir" , "Kind of singing voice, male or female" , "High male singing voice" , "Key old > male who scores high" .)