Possible Clues:

• Like a worthy watchdog
• Red flag, maybe
• Fit for guard duty
• 'Beer will get you on your toes, all right, in short (5)'
• On the lookout for drink at right temperature
• 'Not Guinness? In short, right. Watch out for this (5)'
• Show heads-up
• "Heads up!" e.g
• On your toes
• Thinking or acting quickly