Fencing weapon (4)
I believe the answer is:
foil
'fencing weapon' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for foil that I've seen before include "Parry" , "Frustrate with fencing sword" , "shiny paper" , "Thwart - contrasting thing - sword" , "Food kept fresh in this" .)