Related Clues:

• 'You're mistaken'
• 'Sorry, dude'
• "Ain't happening"
• 'Can't help ya!'
• ''Forget it''
• "Negatory!"
• Gary Cooper's refusal to work in Tyneside
• Denial honest, then back to front
• "Ain't gonna happen!"
• "Guess again"
• "Nuh-uh"
• "Nah!"
• Terse negative
• "You're dreaming"
• Slangy denial, and a hint to 20-, 29-, 46- and 56-Across