Crossword Genius

Be less than terribly wrong (3)

Ross

I believe the answer is:

err

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

'be less than terribly' is the wordplay.
'ERR' is hidden within the letters.
But, I am not sure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for err that I've seen before include "Go wrong" , "Miscalculate" , "Boob" , "Stray from the straight and narrow" , "Bungle" .)

I've seen this clue in the Evening Standard.
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