Theatre masonry styles (5)

I believe the answer is:

opera

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'theatre' is the definition.
(opera is a kind of theatre)

I cannot understand how the rest of the clue works.

(Other definitions for opera that I've seen before include "Don Giovanni, for instance" , "Eg, Rigoletto" , "art form" , "Glyndebourne production" , "musical presentation" .)

I've seen this clue in The Telegraph.

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