Sin or mistake (5)
I believe the answer is:
error
'mistake' is the definition.
(I know that mistake can be written as error)
'sin or' is the wordplay.
'sin' becomes 'err' (to sin is to err morally).
'err'+'or'='ERROR'
(Other definitions for error that I've seen before include "Faulty judgement" , "Slip-up" , "Bloomer, blunder" , "Boo-boo" , "Mistake from terrorist" .)