Thwart, using this sword? (4)
I believe the answer is:
foil
I believe this is a double definition.
'thwart' is the first definition.
(I've seen this before)
'this sword?' is the second definition.
(foil is a kind of sword)
'using' acts as a link.
(Other definitions for foil that I've seen before include "metal used in > sword" , "Thwart - contrasting thing - sword" , "Thin metal - sword" , "Piece of thin and flexible sheet metal" , "Hinder - prevent" .)