Outmoded description of St Peter's for example (5)

I believe the answer is:

domed

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'outmoded' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I can't see?

I cannot understand how the rest of the clue works.

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(Other definitions for domed that I've seen before include "not flat!" , "cupola" , "Shaped like rounded roof" , "with a meniscus" , "Vaulted" .)

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

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