Female in a string quartet (5)

I believe the answer is:

viola

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'string quartet' is the definition.
'viola' can be an answer for 'string' (thesaurus). I am unsure of the 'quartet' bit.

'female in a' is the wordplay.
'female in' becomes 'viol' (I can't justify this - if you can you should believe this answer much more).
'viol'+'a'='VIOLA'

Can you help me to learn more?

(Other definitions for viola that I've seen before include "Large fiddle" , "Bowed instrument slightly larger than a violin" , "Stringed instrument tuned lower than a violin" , "big fiddle!" , "Pansy genus" .)

I've seen this clue in the Evening Standard.

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