Instruct old soldier (8)
I believe the answer is:
commando
'soldier' is the definition.
(I know that commando is a type of soldier)
'instruct old' is the wordplay.
'instruct' becomes 'command' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'command'+'o'='COMMANDO'
Can you help me to learn more?
(Other definitions for commando that I've seen before include "Green Beret" , "Soldier trained to carry out dangerous raids" , "One soldier" , "going this way is unbriefed?" , "Shock troop unit" .)