Make a mistake in the manner reckoned (3)
I believe the answer is:
err
'make a mistake' is the definition.
(to err is to make an error)
'in the manner reckoned' is the wordplay.
'in' indicates a hidden word.
'ERR' is hidden in the letters of 'manner reckoned'.
(Other definitions for err that I've seen before include "Be mistaken" , "Blunder" , "Boob" , "Go astray" , "Be incorrect" .)