Possible Clues:

• Inescapable attack in chess
• Chess victory
• Staunch friend's defeat
• To spy on one's partner is the end
• Final chess move
• The killer move in chess
• Grandmaster's last word ... and what the last word in 17-, 24-, 36- and 52-Across can be
• The final defeat of the board
• Lioness in winter?
• The final killer move in chess