Related Clues:

• Notify of danger
• On the ball
• Like a good guard
• 'Heads up!,' e.g.
• Bouncing off the walls
• See 52-Down
• Heads-up
• Argus-eyed
• Watchful
• Whistle, for one
• Warn
• Vigilant
• See 10-Down
• Itchy
• On one's toes