Related Clues:

• Quick on the uptake
• Attentive
• With ears pricked
• Not nodding
• Red flag, e.g.
• Prepared for action
• Caffeinated?
• Trouble indicator
• Not dozing
• Not nodding
• 'Look out below!,' e.g.
• Awake and aware
• Having pricked ears
• Keen-eyed
• Like watchmen