An error in finishing up (3)
I believe the answer is:
sin
'an error' is the definition.
(I've seen this in other clues)
'in finishing up' is the wordplay.
'in' says the answer is hidden in the clue.
'up' is a reversal indicator (in down clue: letters go upwards).
'nis' is hidden within 'finishing'.
'nis' in reverse letter order is 'SIN'.
(Other definitions for sin that I've seen before include "Fall from grace" , "Transgress" , "Err" , "trip" , "Serious offence" .)