Opera and musical in back-to-back clash, setting a standard (9)

I believe the answer is:

normative

'setting a standard' is the definition.
(setting a standard or norm)

'opera and musical in back-to-back clash' is the wordplay.
I cannot quite see how this works, but
'opera' could be 'norma' (I've seen this in another clue) and 'norma' is found within the answer.
This explanation may well be incorrect...

(Another definition for normative that I've seen is " setting a standard".)

I've seen this clue in The Times.

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