Arch members like Sennet's cops (9)

I believe the answer is:

keystones

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'cops' is the definition.
Although both the answer and definition are plural nouns, I cannot see how they can define each other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Another definition for keystones that I've seen is " They are central to arches".)

I've seen this clue in the Sydney Morning Herald.

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