Thin sword (6)
I believe the answer is:
rapier
'thin sword' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rapier that I've seen before include "A small spring flower" , "Straight sword with narrow blade" , "Fine-bladed sword" , "Thrusting weapon" , "Straight narrow sword with two edges" .)