Be less than terribly wrong (3)
I believe the answer is:
err
'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" .)