Prayer for old prison head departing (6)

I believe the answer is:

orison

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

I can't explain the rest of the clue.

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

I've seen this clue in The Irish Times.

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