Frustrate using sword (4)
I believe the answer is:
foil
This clue is a double definition.
'frustrate' is the first definition.
(I know that frustrate can be written as foil)
'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 "Fencing rapier" , "Thwart - contrasting thing - sword" , "Frustrate - sheet - rapier" , "metal used in > sword" , "Leaf of metal" .)