Related Clues:

• Champing at the bit
• *Siren ... danger ... perked up
• Word before 'set, go!'
• Warning
• Part of a willing trio
• Perceptive
• Sound the tocsin
• Disposed
• Encircled
• On your toes
• With eyes wide open
• Mentally responsive
• It may be red
• Qui vive
• All ears