Possible Clues:

• Final zinger
• End of a testy exchange, and what can be found four times in this puzzle, thanks to four black squares
• 22 Across moving right on past?
• Final cross words
• End of a heated exchange, perhaps
• Cutting remark on past right to be transferred
• Phrasing OTT possibly for this final comment?
• Final retort
• Bit of one-upmanship ... and what can be found at the end of 18-, 24-, 39- and 51-Across?
• Bullet that meant goodbye for someone?