1. Grate rind from half of orange to measure 2 teaspoons. Cut orange half into quarters. Combine orange rind, butter, and next 4 ingredients (through garlic) in a small bowl. (Reserve remaining orange half for another use.)
2. Remove and discard giblets and neck from chicken. Trim excess fat. Starting at neck cavity, loosen skin from breast and drumsticks by inserting fingers, gently pushing between skin and meat. Rub half of butter mixture under loosened skin. Rub remaining butter mixture over surface of chicken. Lift wing tips up and over back; tuck under chicken. Place orange pieces in cavity.
3. Place chicken, breast side up, on a small rack coated with cooking spray. Place rack inside a 5-quart electric slow cooker.
4. Cover and cook on LOW for 4 hours or until a thermometer inserted into meaty part of thigh registers 165.
5. Remove chicken from slow cooker. Let stand 15 minutes. Discard skin.