Related Clues:

• Utter '...or else!'
• Admonish
• Cry 'Heads up!' to
• Fencing term
• Put on guard
• Fully alert
• Give a heads-up to
• Raise red flags
• Notify of potential danger
• Give a heads-up
• Issue a caveat
• Give an alert
• Sound the alarm
• Notify in advance
• Honk at, perhaps