Royals to make a mistake (3)

I believe the answer is:

err

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'make a mistake' is the definition.
(to err is to make an error)

I don't understand the rest of the clue.

(Other definitions for err that I've seen before include "Slip up" , "'To . . . is human, to forgive divine'" , "do something you shouldn't" , "Make a wrong move" , "Drop a clanger" .)

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

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