Opera reduced to scale (5)

I believe the answer is:

tosca

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

'opera' is the definition.
(Giacomo Puccini opera)

'reduced to scale' is the wordplay.
I cannot quite see how this works, but
'to' is within the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for tosca that I've seen before include "role for diva" , "Puccini's jumper" , "musical work" , "Opera - a Scot (anag)" , "opera singer" .)

I've seen this clue in the Evening Standard.

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