Posted like a soldier of old (6)
I believe the answer is:
mailed
'posted' is the definition.
('mail' can be a synonym of 'post')
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for mailed that I've seen before include "Posted letters with armoured fist?" , "Covered with armour or sent by post" , "Posted - wearing armour?" , "Armoured, or posted in America" .)