Viola's singing voice (4)

I believe the answer is:

alto

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This clue might be a double definition.

'viola's singing' is the first definition.
'alto' can be an answer for 'singing' (thesaurus). I am unsure of the 'viola's' bit.

'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 "Highest male voice: lowest female voice" , "Counter-tenor or contralto" , "Unisex vocal range" , "someone to sing" , "Singer's hidden inside chemical toilet" .)

I've seen this clue in the Evening Standard.

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