Sin initially expiated by bishop (3)
I believe the answer is:
err
'sin' is the definition.
(to sin is to err morally)
'initially expiated by bishop' is the wordplay.
'initially' indicates taking the first letters.
'by' is a charade indicator (letters next to each other).
'bishop' becomes 'rr' (abbreviation for Right Reverend).
The first letter of 'expiated' is 'e'.
'e'+'rr'='ERR'
(Other definitions for err that I've seen before include "Blunder" , "Drop a clanger" , "do something you shouldn't" , "'to... is human,to forgive divine'" , "Be inaccurate" .)