Possible Clues:

• It's thought to be a grand one
• "Clever thought!"
• "Nice thinking!"
• 'Why didn't I think of that?!'
• "That's genius!"
• "I love that suggestion!"
• "Wish I'd thought of that!"
• 'Wish I'd thought of that!'
• 'That's a smart thought!'
• Brainwave said in response to 'what goes with a grey suit, darling?'