Soldier's right: a pistol backfired (7)
I believe the answer is:
regular
'soldier's' is the definition.
(member of the regular army)
'right a pistol backfired' is the wordplay.
'right' becomes 'r' (common abbreviation).
'pistol' becomes 'luger' (I have seen 'German pistol' mean 'luger' so perhaps 'pistol' could also mean 'luger').
'backfired' says the letters should be written in reverse.
'r'+'a'+'luger'='raluger'
'raluger' back-to-front is 'REGULAR'.
(Other definitions for regular that I've seen before include "Perishes" , "Serviceman" , "Symmetrically arranged" , "Career soldier" , "Predictable" .)