Army securing large essentially superior case for weapon (7)
I believe the answer is:
holster
'case for weapon' is the definition.
'army securing large essentially superior' is the wordplay.
'army' becomes 'host' (host is a kind of army).
'securing' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'essentially' means to look at the middle letters.
The middle letters of 'superior' are 'er'.
'host' going around 'l' is 'holst'.
'holst'+'er'='HOLSTER'
(Other definitions for holster that I've seen before include "Pistol case" , "Leather case" , "Gun-case" , "Receptacle for a hand-gun" , "Pistol-case" .)