Related Clues:

• Itch
• Chew
• Whim
• Drive
• Driving force
• Urge
• Impetus
• Instinctive motive
• Catalyst
• Off-the-cuff response … and a hint to the answers to the four starred clues
• Knee-jerk response, perhaps
• Thrust
• Sudden urge
• Action stimulus
• Sudden desire