Thwart with a sword (4)
I believe the answer is:
foil
I believe this clue is a double definition.
'thwart' is the first definition.
(I know that thwart can be written as foil)
'sword' is the second definition.
(foil is a kind of sword)
'with a' is the link.
(Other definitions for foil that I've seen before include "Queer" , "Piece of thin and flexible sheet metal" , "Leaf of metal" , "Prevent the success of; fencing sword" , "metal used in > sword" .)