Possible Clues:

• 'Absolutely not!'
• Opposite of 30-Down
• 'When pigs fly!'
• Not on your life
• "Forget it!"
• Absolutely not
• 'Just say '
• Thumbs-down
• With 38-Across, what a two-letter answer is in a crossword, usually ... or a hint to 20-, 24-, 44- and 51-Across
• See 35-Across