Some blood not English: a simple mistake? (4)

I believe the answer is:

typo

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'mistake?' is the definition.
(typographical error)

I can't explain the rest of the clue.

(Other definitions for typo that I've seen before include "Literal error in printed matter, in short" , "Spelling mistake" , "Finger problem" , "Printing error (colloq.)" , "Character flaw" .)

I've seen this clue in The Times.

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