Foundation garment (6)
'foundation garment' is the definition.
(I know that corset is a type of foundation garment)
This is the entire clue.
(Other definitions for corset that I've seen before include "A woman's close-fitting support undergarment" , "female garment!" , "Female undergarment" , "Tight undergarment" , "Stiffened undergarment" .)
(Other definitions for girdle that I've seen before include "Female undergarment" , "Close-fitting foundation garment" , "CLOTHING" , "Waistbelt" , "historic zone" .)