Double-edged sword (6)
I believe the answer is:
rapier
'double-edged sword' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rapier that I've seen before include "Thrusting weapon" , "Narrow sword with two-edged blade" , "Straight narrow sword with two edges" , "Light sword" , "A thin double-edged sword" .)