Possible Clues:

• After end of play stage will be left so dull?
• Boring teacher struggling with class — headmaster finally intervenes
• Dull, uninteresting
• Flat not having received letter?
• Dull, like a blank sheet of paper?
• Bland heir to the throne�s gripping performance -- monarch
• Unlettered and lacking personality?
• Lacking distinctive qualities
• Lacking a distinctive nature
• Dull person leading session for pupils? That's not on!