Mistake blood group in report (4)

I believe the answer is:

typo

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'mistake' is the definition.
(typographical error)

I cannot understand how the rest of the clue works.

(Other definitions for typo that I've seen before include "Printing error (colloq.)" , "Thanks; good health!" , "Keyboard slip-up" , "What's not right" , "Finger problem" .)

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