Soldier takes up artillery weapon (7)
I believe the answer is:
regular
'soldier' is the definition.
(member of the regular army)
'up artillery weapon' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'artillery' becomes 'RA' (Royal Artillery).
'weapon' becomes 'luger' (I've seen this in other clues).
'ra'+'luger'='raluger'
'raluger' written backwards gives 'REGULAR'.
'takes' acts as a link.
(Other definitions for regular that I've seen before include "Uniform, even" , "Recurring at short uniform intervals" , "Soldier - uniform" , "Career soldier" , "Full-time soldier" .)