Cut 1/2-inch-thick horizontal slice from top of cake. Make a cavity in loaf by removing cake from center, leaving 1/2-inch shell on 4 sides and bottom. (Save cake pieces for a later use.)
In food processor bowl with metal blade, combine orange slice, cranberries and sugar. Process until finely chopped; do not puree.
Spoon sherbet into medium bowl. Add cranberry-orange mixture; fold in until well blended. Spoon sherbet mixture into cake cavity. Replace top slice of cake. Wrap cake in foil; freeze about 4 hours or until firm.
Remove loaf from freezer 20 minutes before serving. To serve, cut loaf into slices.