Related Clues:

• With 21-Across, begin from scratch
• Make a start
• Outset
• Machine button
• Turn a key, perhaps
• Word next to an arrow in a maze
• One end of a maze
• "___ the Revolution Without Me" (1970)
• Zero hour
• Get rolling
• Involuntary jerk
• Step one
• Proctor's instruction
• Jump into
• Test proctor's command