Long slender sword (6)
I believe the answer is:
'long 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 "Fine-bladed sword" , "A thin double-edged sword" , "Sword for thrusting" , "Straight narrow sword with two edges" , "Long slender sword" .)