Thwart with sword (4)
I believe the answer is:
foil
I believe this clue is a double definition.
'thwart' is the first definition.
(I've seen this before)
'sword' is the second definition.
(foil is a kind of sword)
'with' acts as a link.
(Other definitions for foil that I've seen before include "Scotch" , "Thwart - contrasting thing - sword" , "Blunt sword with a button on the end" , "Queer" , "Piece of thin and flexible sheet metal" .)