Related Clues:

• On the ball
• Start of a Christmas carol
• Like a good guard
• Carol start
• 'Heads up!,' e.g.
• 'Hey. You. Yeah, you!'
• 'Hey, bud!'
• See 52-Down
• 'Hey, there!'
• Heads-up
• Argus-eyed
• Watchful
• 'Check this out!'
• Whistle, for one
• Warn