Drain clams, and put juice on aside. In a skillet, saute onion in 1 tbsp. of butter until tender. Stir in flour until blended. Gradually add horseradish, garlic powder, Worcestershire sauce, salt and reserved clam juice. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from heat; stir in clams. Flatten bread with a rolling pin. Melt remaining butter; brush one side of each slice of bread. Spread with clam mixture; roll up. Brush with remaining butter; sprinkle with paprika. Cut rolls into thirds; place on a greased baking sheet. Bake at 425 degrees for 5-8 minutes or until lightly browned. Serve warm.