Petticoat, a mistake (4)
I believe the answer is:
slip
I believe this is a double definition.
'petticoat' is the first definition.
(thesaurus)
'a mistake' is the second definition.
(slip is a kind of mistake)
(Other definitions for slip that I've seen before include "release > voucher" , "failure" , "Slide; petticoat" , "Loose one's footing" , "Mistake; garment" .)