Related Clues:

• Caveat
• Watch out for
• Signal for help
• Warning word
• 'Accident ahead' indicator
• Become wider
• Caveat starter
• Suddenly intensify, with "up"
• Burst of light
• Be cautious
• Roadside alert
• Distress signal shot into the air
• Bell-bottoms feature
• Word on a dog-owner's sign, perhaps
• 'Heads up!'