Using a vegetable peeler, remove 2 strips of zest from the orange. Thinly slice the zest. Juice the orange into a 5- to 6-qt slow cooker (you should get about 1/2 cup). Whisk in the sugar, vinegar, 3/4 teaspoon salt, and 1/4 teaspoon pepper.
Toss the chicken with the flour, then add to the slow cooker along with the garlic and orange zest. Scatter the onion over the top. Cook, covered, until the chicken is cooked through and the sauce has slightly thickened, 7 to 8 hours on low or 5 to 6 hours on high.
Thirty minutes before serving, cook the rice according to package directions.
When the rice is nearly finished cooking, transfer the chicken to plates or a platter. Add the peaches to the slow cooker and cook, covered, until heated through, about 3 minutes; fold in the parsley. Spoon the peach and onion mixture over the chicken and serve with the rice.