Soldier's gun returned by a king (7)
I believe the answer is:
regular
'soldier's' is the definition.
(member of the regular army)
'gun returned by a king' is the wordplay.
'gun' becomes 'luger' (type of pistol).
'returned' shows that the letters should be reversed in order.
'by' means one lot of letters go next to another.
'king' becomes 'r' (r is abbreviation for rex).
'luger' back-to-front is 'regul'.
'regul'+'a'+'r'='REGULAR'
(Other definitions for regular that I've seen before include "Usual; done frequently" , "Customary; done frequently" , "Frequent - usual" , "orderly" , "Predictable" .)