Slender sword (6)
I believe the answer is:
rapier
'slender sword' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rapier that I've seen before include "Light sword" , "fencing equipment" , "A thin double-edged sword" , "Straight sword with narrow blade" , "Sword for thrusting" .)