Rex, on a dock, finds sword (6)
I believe the answer is:
rapier
'sword' is the definition.
(rapier is a kind of sword)
'rex on a dock' is the wordplay.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'on' says to put letters next to each other.
'dock' becomes 'pier' (I've seen this before).
'r'+'a'+'pier'='RAPIER'
'finds' acts as a link.
(Other definitions for rapier that I've seen before include "Light sword" , "A small spring flower" , "Narrow sword with two-edged blade" , "Straight sword with narrow blade" , "Sword for thrusting" .)