Ring in right hours following old church service (7)

I believe the answer is:

orthros

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'ring' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's an association between them I don't understand?

I don't understand the rest of the clue.

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(Other definitions for orthros that I've seen before include "Time for prayer in Greece!" , "equivalent to lauds" .)

I've seen this clue in The Sunday Times and The Times.

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