Piece of material used to enlarge a garment (6)
I believe the answer is:
gusset
'piece of material used to enlarge a garment' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gusset that I've seen before include "Guests (anag.)" , "Piece of material set into a dress" , "Piece of inserted material" , "Gets us (anag.)" , "Garment enlarger" .)