Possible Clues:

• Sounds as if I got some money from you for the mercenary
• Join, in a way, when one gets admitted in private
• Maybe, private institute? Join at the margins
• One went in to join up
• Person serving in army
• Fighter's slice of bread and butter
• Company man?
• Bond — one retained as trained killer
• Serving person a piece of toast
• One serves in the army