Landing stage needed to support artillery weapon (6)
I believe the answer is:
rapier
'weapon' is the definition.
(rapier is a kind of weapon)
'landing stage needed to support artillery' is the wordplay.
'landing stage' becomes 'pier' (structure by the sea).
'needed to support' is a charade indicator (letters next to each other) (in a down clue, letters underneath support others).
'artillery' becomes 'RA' (Royal Artillery).
'pier' put after 'ra' is 'RAPIER'.
(Other definitions for rapier that I've seen before include "A fencing sword for repair" , "Narrow sword with two-edged blade" , "A thin double-edged sword" , "Fine-bladed sword" , "Straight sword with narrow blade" .)