Related Clues:

• 'Sorry, it's impossible for me'
• "That is just TOO much for me!"
• Busy person's response to an invitation
• "Sorry, too busy" (In this clue's answer, note the last 3 letters + the first 3 of 21-Across)
• "Today's a no-go for me"
• "It's all too much"
• "That's not possible for me"
• "Totally impossible"