Weapon gives gunners massive support (6)
I believe the answer is:
rapier
'weapon' is the definition.
(I know that rapier is a type of sword)
'gunners massive support' is the wordplay.
'gunners' becomes 'RA' (Royal Artillery).
'massive support' becomes 'pier' (large structure).
'ra'+'pier'='RAPIER'
'gives' is the link.
(Other definitions for rapier that I've seen before include "Narrow sword with two-edged blade" , "Sword for thrusting" , "Fine-bladed sword" , "Straight sword with narrow blade" , "Straight narrow sword with two edges" .)