Related Clues:

• Word next to an arrow, maybe
• Stunned sound
• Possible result of a gunshot
• Boot up
• With 21-Across, begin from scratch
• Inhale in horror
• Hit back, say
• Undergo chemical change
• Not keep a poker face
• Express shock or surprise
• Outset
• Machine button
• Last ___
• Duck, say
• Recover from a sprint, perhaps