Mistake this for an undergarment (4)
I believe the answer is:
slip
I believe this is a double definition.
'mistake' is the first definition.
(a slip is a blunder or mistake)
'for an undergarment' is the second definition.
(slip is a kind of undergarment)
'this' acts as a link.
(Other definitions for slip that I've seen before include "Careless mistake" , "release > voucher" , "Mistake (of tongue or pen)" , "Man in field" , "Slide; petticoat" .)