Related Clues:

• Red flag
• See 42-Down
• Poe visitor
• Ready for anything
• Active
• Notify of danger
• On the ball
• Like a good guard
• Mystery writer's award
• 'Heads up!,' e.g.
• See 52-Down
• Heads-up
• Poe called it 'grim, ungainly, ghastly, gaunt and ominous'
• Argus-eyed
• Watchful