Sin or slip (5)
I believe the answer is:
error
This clue is a double definition.
'sin' is the first definition.
(I've seen this in another clue)
'slip' is the second definition.
(slip can mean an error or mistake)
(Other definitions for error that I've seen before include "Bloomer, blunder" , "heresy" , "Blunder, slip" , "Slip, mistake" , "wrongdoing" .)