Music composed not as a variation (6)

I believe the answer is:

sonata

I'm a little stuck... Click here to teach me more about this clue!

'music' is the definition.
(I know that sonata is a type of classical)

'composed not as a variation' is the wordplay.
'not as a' anagrams to 'SONATA'.
Though, I'm not clear how this is indicated.

Can you help me to learn more?

(Other definitions for sonata that I've seen before include "To NASA, strangely, for the solo piece" , "Piece of music for solo piano" , "Classical composition" , "Solo instrumental composition" , "Music to play (not sing)" .)

I've seen this clue in The Globe and Mail.

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