Eastern bishop's sin (3)
I believe the answer is:
err
'sin' is the definition.
(to sin is to err morally)
'eastern bishop's' is the wordplay.
'eastern' becomes 'e'.
'bishop' becomes 'rr' (abbreviation for Right Reverend).
'e'+'rr'='ERR'
(Other definitions for err that I've seen before include "Stray from the straight and narrow" , "Blunder" , "Miscalculate" , "'To . . . is human, to forgive divine'" , "Make a wrong move" .)