Rex, on a jetty, finds sword (6)
I believe the answer is:
rapier
'sword' is the definition.
(rapier is a kind of sword)
'rex on a jetty' is the wordplay.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'on' means one lot of letters go next to another.
'jetty' becomes 'pier' (I've seen this before).
'r'+'a'+'pier'='RAPIER'
'finds' is the link.
(Other definitions for rapier that I've seen before include "Thrusting weapon" , "fencing equipment" , "Straight narrow sword with two edges" , "Light sword" , "Thin sharp sword" .)