Possible Clues:

• It's overheard at roadblock that it has a supersonic quality
• Strong defence, yet speedily broken
• A supersonic jet may break it (2 words)
• Invisible limit of a sense
• It's responsible for a boom in flying
• Reliable fence needs to be approached by high-flyers looking for speed
• What 69 Across broke in October 1947 ... and what you must break nine times to reveal this puzzle's hidden phrase
• Uninjured, rare bird almost crashed into something met at high speed
• Air resistance Scottish writer found in channel run