Stray from proper rectitude (3)
I believe the answer is:
err
'stray' is the definition.
(to err is to stray or go wrong)
'from proper rectitude' is the wordplay.
'from' indicates the answer is hidden within the clue.
'ERR' is hidden within 'proper rectitude'.
(Other definitions for err that I've seen before include "Leave the straight and narrow" , "do something you shouldn't" , "Be mistaken" , "Boob" , "Make a wrong move" .)