A mistake in the paper (4)
I believe the answer is:
slip
I believe this is a double definition.
'a mistake' is the first definition.
(a slip is a blunder or mistake)
'paper' is the second definition.
(slip is a kind of paper)
'in the' acts as a link.
(Other definitions for slip that I've seen before include "shift" , "Silly little mistake" , "release > voucher" , "Cricket fielder - mistake" , "pre-printed form" .)