Decree otherwise and I fall out with (6)

I believe the answer is:

ordain

I'm a little stuck... Click here to teach me more about this clue!

'decree' is the definition.
(I know that decree is the is a more specific form of the action of ordain.)

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

Can you help me to learn more?

(Other definitions for ordain that I've seen before include "Make a decree" , "Make someone a clergyman" , "Elevate or appoint to priesthood" , "Decree and admit to holy orders" , "Establish" .)

I've seen this clue in the Evening Standard.

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