A prayer of three short words (6)

I believe the answer is:

orison

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'a prayer' is the definition.
(I know that prayer can be written as orison)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for orison that I've seen before include "Prayer - O no sir! (anag)" , "An archaic or literary prayer" , "Supplication" , "O, no, sir, it makes a prayer" .)

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

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