Combine the first six ingredients in a slow cooker.
Cut the chicken breasts in slices, and season with five spice powder, salt and pepper.
Place chicken slices in the slow cooker and stir the chicken a little bit so it is covered with the sauce and onions. Top with clementines, cover and cook at LOW for 4 hours.
Remove chicken and onions from slow cooker. Pour liquid in a medium saucepan, reserving about 1/4 cup. Mix reserved liquid with 1 1/2 Tbsp arrowroot starch, until smooth. Return to the orange mixture in the saucepan and bring to a boil. Boil, stirring constantly, 1 minute or until sauce thickens.
Serve sauce over chicken, sprinkle with pomegranates or green onions, if desired.