Make your breakfast campfire nice and hot. You have, of course, built your fire in a safe, deepish hollow. Get the clay mud from nearby (loam or sand doesn’t work) and have it ready. Cook and eat your breakfast. Wash your hands.
You’ve already cleaned your duck, getting rid of the offal, so that’s done. Wipe the inside out well with a clean cloth, first, then do the outside, too (you don’t want stuff in your food).
Rub the inside of the duck thoroughly with salt and a bit of pepper.
Clean the apple and onion. Stuff them both inside the duck’s cavity.
Fold the feathers to cover all the openings in the duck, then plaster the entire bird with the clay mud until it's about 1 inch thick.
Place the duck in the ashes in the campfire pit and cover it well with some coals and a generous amount of wood (but don’t get carried away).
Make sure the campfire is safely away from everything flammable, safely banked. (If you have someone staying back at camp, that works, too.) Then go fishing or hunting for the day.
When you come back for dinner, dig the duck out of the ashes (it should still be hot) and break off the hardened clay. The feathers will come off with the mud.
Pull off any remaining feathers, and eat your delicious duck!