Defeat with a sword (4)
I believe the answer is:
foil
This clue is a double definition.
'defeat' is the first definition.
(thesaurus)
'sword' is the second definition.
(I know that foil is a type of fencing sword)
'with a' is the link.
(Other definitions for foil that I've seen before include "Food kept fresh in this" , "Check" , "Baffle" , "Parry" , "Prevent the success of; fencing sword" .)