Give orders to ordinary Royal Marine (8)
I believe the answer is:
commando
'royal marine' is the definition.
'commando' can be an answer for 'marine' (I've seen this before). I'm not certain of the 'royal' bit.
'give orders to ordinary' is the wordplay.
'give orders to' becomes 'command' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ordinary' becomes 'O' (abbreviation e.g. O-Levels).
'command'+'o'='COMMANDO'
Can you help me to learn more?
(Other definitions for commando that I've seen before include "going this way is unbriefed?" , "specially-trained soldier" , "Army unit" , "Soldier trained for raiding" , "One soldier" .)