Slip or sin (5)
I believe the answer is:
lapse
This clue is a double definition.
'slip' is the first definition.
(I know that lapse is a more specific form of the action slip)
'sin' is the second definition.
(I've seen this before)
(Other definitions for lapse that I've seen before include "Decline into an inferior state" , "Slight error as caused by inattention" , "Brief failure (of concentration)" , "Loss (in concentration, say?)" , "Momentary mistake" .)