Possible Clues:

• Finishing cake, a little dyspeptic in guts
• Deposit engendering some panic in Gatwick?
• Bonus to something that's already good
• Pastry bag fill
• Frosting on the cake
• They take the form of self-flying paper airplanes in the Harry Potter books
• Topping in a bag
• Sugary coat on a cake
• Creamy concoction
• Reportedly I spill the beans on the cake?