Army officer's weapon prepared around range when training (5,8)
I believe the answer is:
lance sergeant
'army officer's' is the definition.
(military rank)
'weapon prepared around range when training' is the wordplay.
'weapon' becomes 'lance' (lance is a kind of weapon).
'prepared' becomes 'set' (set up or ready).
'around' indicates putting letters inside.
'when training' indicates an anagram (to train can mean to grow something into a shape).
'range' with letters rearranged gives 'rgean'.
'set' going around 'rgean' is 'sergeant'.
'lance'+'sergeant'='LANCE-SERGEANT'
(Other definitions for lance sergeant that I've seen before include "Officer" , "Soldier" .)