Crossword Genius

Mistake that’s terrible (doubly so!) (3-3)

Ross

I believe the answer is:

boo-boo

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'mistake' is the definition.
(I know that boo-boo is a type of mistake)

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

Can you help me to learn more?

(Other definitions for boo-boo that I've seen before include "Clanger" , "Mistake" .)

I've seen this clue in The Times.
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