Related Clues:

• On the ball
• Like a good guard
• 'Heads up!,' e.g.
• See 52-Down
• Heads-up
• Argus-eyed
• Watchful
• Whistle, for one
• Warn
• Vigilant
• See 10-Down
• On one's toes
• Like Argus
• Sentry's attribute
• Ready to react, perhaps