Possible Clues:

• Fall on
• So silly to get sick while yachting, like
• Something like yachting to make the silly one sick
• Attack a number of ships positioned around Spithead
• Attack that may make the silly one sick
• So to 25 across is enough to make the silly sick
• Sounds as if an attack of 10 across may make a silly one sick
• Fall upon
• Attack Christmas festivity that's not opened
• Maybe mug or fool put ahead of excellent learner