Related Clues:

• Qui vive
• Flag, maybe
• All ears
• A wink or a nod, e.g.
• Wide-awake
• Ready to jump, perhaps
• On guard
• Wary
• Whistle, maybe
• At the ready
• Far from sleepy
• Bright-eyed and bushy-tailed
• Advance warning
• Watching
• Quick on the uptake