Possible Clues:

• Rule out major key in opening music
• 'Prevent, forestall (8)'
• Sounds as if the clue to this was made beforehand
• Rule out chapter in introduction
• Prevent, forestall the possibility of
• In the overture, a note and bar
• Rule out mention of speed of light in introduction
• Prevent from happening
• Prevent (something) from happening
• Rule out or eliminate