Possible Clues:

• Communication that encodes a message
• Indication or warning
• Gesture, light or sound which sends a message
• A wink or a nod, say
• Nod, at auctions
• Remarkably good or bad; transmit (message)
• Gesture important to train drivers striking
• Communicate non-verbally with a student - that's impressive
• Communication with encoded message
• Warning symptom with animal skin