Place chicken breast halves between 2 pieces of waxed paper. Pound chicken to 1/8" thickness. Mix sugar and cinnamon. Coat apple slices with sugar mixture. Divide apples among chicken breast halves; secure with toothpicks. Cover and grill chicken 4-6" from medium coals, 20-25 minutes, turning after 10 minutes, until done. Remove toothpicks. Mix the apple cider and cornstarch in 1 quart saucepan. Cook over medium heat until thickened and bubbly. Spoon over chicken.