Fat cello playing? This is for viola! (4,4)

I believe the answer is:

alto clef

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'for viola' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'fat cello playing?' is the wordplay.
'playing?' indicates an anagram.
'fat'+'cello'='fatcello'
'fatcello' anagrammed gives 'ALTO CLEF'.

'this is' acts as a link.

Can you help me to learn more?

(Another definition for alto clef that I've seen is " Stave marking".)

I've seen this clue in The Guardian.

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