Stray from proper rectitude (3)
I believe the answer is:
err
'stray' is the definition.
(synonyms)
'from proper rectitude' is the wordplay.
'from' indicates a hidden word.
'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" .)