Possible Clues:

• Might the earth be able to get the wind up?
• During swim, Royal Engineers use aqualungs, perhaps
• Soldiers infiltrate city and there finally expire
• Take air in and out of the lungs
• That's the way to get the end of 11 across
• With quiet intensity, say b— heater's broken
• Respire — inspire — expire
• Take inspiration from book about what some soldiers turn up
• Inspire soldiers to swim outside
• "Just relax!"