Possible Clues:

• Fliers with lots of guts
• Planes' first fliers, often
• Risky high flyers providing proof by showing off pistol
• Firsts in flight
• Risky high flyers in 10 across showing off pistol
• Aviators trying out new planes
• First four characters in 6 down freely spoilt those with their heads in the clouds in the study
• Risky high-flyers in 10 across at institute surrounded by conspiracies
• First-time fliers slept fitfully in shorts, I gathered
• Each trial is a turn-up for them