Preheat oven to 350 degrees F. Warm a little clarified butter in casserole dish on stovetop over high heat. Add pineapple to cover base. Cover pineapple with neck and giblets and dribble in any juices from pheasant. Chop wings and add to dish. Cook, turning pineapple to color on both sides.
Season inside and outside of bird with salt and pepper. Truss bird and place on top of ingredients in casserole. Cover and cook for 5 minutes on high heat. Loosen any stuck pieces of pineapple and pour in pineapple juice. Brush bird with clarified butter and place in oven, uncovered, for 1 hour. Baste during cooking to keep bird moist. After 1 hour, cover bird with brown paper or buttered parchment paper and cook another 20 minutes.
Remove bird from casserole. Skim off top layer of fat juices, and strain dark residue collected in pan to a separate small pot. Bring this to a boil. Blend arrowroot and 1/2 cup wine. Add this to pot, along with remaining wine. Boil until arrowroot thickens and clears.
Carve pheasant and transfer to heated serving platter. Coat pieces with sauce and garnish with watercress and potatoes.