Place in 4 qt. or larger slow cooker. With a fork prick skin of duckling all over at approximately 2" intervals.
Place duck on rack in slow cooker. (When cooking duckling in a 3 1/2 qt. or smaller slow cooker, cut the duckling into quarters or halves before putting it into the pot.)
In a small saucepan, combine onion, tarragon, orange juice, salt, mustard, jelly, orange peel, wine and cornstarch. Cook over med. heat until thickened. Brush 1/3 cup sauce over duckling, reserving remaining sauce. Cover and cook on LOW 6 to 7 hrs. or until duckling is tender, turning once during cooking. If possible, remove fat with a bulb baster. Stir orange sections into remaining sauce, heat and pour over duck just before serving.