Related Clues:

• 'Look out below!,' e.g.
• Awake and aware
• Having pricked ears
• Grow paler
• Keen-eyed
• Like watchmen
• Paying attention
• Gradually disappear
• Like sentries
• Film technique
• Like an excellent guard dog
• Grow faint
• On top of things
• Not drowsy
• Word with red or on