Old ways in which soldiers brought back commands (6)
I believe the answer is:
orders
'commands' is the definition.
(I know this)
'old ways in which soldiers brought back' is the wordplay.
'old' becomes 'o' (abbreviation).
'ways' becomes 'rds' ('rd' is an abbreviation for 'road').
'in which' means one lot of letters goes inside another.
'soldiers' becomes 're' (Royal Engineers).
'brought back' says the letters should be written in reverse.
're' in reverse letter order is 'er'.
'rds' going around 'er' is 'rders'.
'o'+'rders'='ORDERS'
(Other definitions for orders that I've seen before include "Commands and edicts" , "Holy - -" , "Dictates, commands" , "Directs" , "Demands" .)