The term ONE ON A MISSION can be summarized by two crossword solutions: CRUSADER (8 letters) and DIPLOMATIC ENVOY (15 letters). A CRUSADER is someone who is passionate about a cause and is willing to fight for it. Historically, crusaders were warriors who fought in the Middle Ages to reclaim the Holy Land from Muslim rule. A DIPLOMATIC ENVOY is a representative of a government or organization who is sent to another country to negotiate agreements or treaties. They are also responsible for maintaining DIPLOMATIC relations between countries.