Related Clues:

• Fallback option
• Plumber's challenge
• Show clemency
• Loan
• Block up
• U-boat
• Verne's lunch?
• Sink stoppage
• What black and white squares do on a chessboard
• Reprint
• Grinder's kin
• Excess
• Fallback idea
• Radio response
• ''Red October,'' for one