Related Clues:

• See 50-Across
• Tip-off
• Alarm
• See 26-Down
• Cautionary advice
• Heads-up
• Blinking light, maybe
• Alternative to a ticket
• Caveat
• See 1 Across
• Message informing of danger
• Caution - threat
• Declining to cross river, showing caution
• All ----- Still (Ultravox)
• Alerting a fleet during flight