On landing stage, artillery weapon (6)
I believe the answer is:
rapier
'weapon' is the definition.
(rapier is a kind of weapon)
'on landing stage artillery' is the wordplay.
'on' says to put letters next to each other.
'landing stage' becomes 'pier' (I've seen this before).
'artillery' becomes 'ra' (Royal Artillery).
'pier' put after 'ra' is 'RAPIER'.
(Other definitions for rapier that I've seen before include "Straight narrow sword with two edges" , "Straight sword with narrow blade" , "Long slender sword" , "Thin sharp sword" , "Sword for thrusting" .)