Underwear gaffe (4)
I believe the answer is:
slip
I believe this clue is a double definition.
'underwear' is the first definition.
(I've seen this before)
'gaffe' is the second definition.
(synonyms)
(Other definitions for slip that I've seen before include "Error - fielder" , "Mistake -- small piece of paper" , "Slide; petticoat" , "Gaffe" , "See 16" .)