Possible Clues:

• The turn of a doctor to act, having swallowed steak: a real advantage
• *Source of a decisive advantage
• Advantage, from the game of bridge
• Secret advantage
• Secret weapon
• Initially trailing behind, character makes winning move
• Key resource to be used at the right moment
• Winning advantage
• Decisive advantage, one blown by eccentric
• Decisive resource of odd copper breaking up bother