Crossword Genius

An error in finishing up (3)

Ross

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" .)

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