Related Clues:

• Leave a little extra
• It may be 20%
• Baksheesh
• Piece of advice
• See 38-Down
• Trackside offer
• Good lead
• Not stiff?
• 15 percent, often
• You can bet on it
• Diner's prerogative
• It's left behind in good restaurants
• Line on a bill
• Visible part of an iceberg
• Something helpful