1. In a large saucepan over medium heat, warm vegetable oil. Add onion, celery and carrots to pan. Cook, stirring often, until softened, about 7 minutes.
2. Sprinkle in flour and cook for 2 minutes, stirring vigorously. Add 4 cups water and reserved clam juice. Bring to a boil, stirring constantly. Stir in tomato paste, tomatoes, potatoes, crushed red pepper and Italian seasoning; season with salt and pepper. Reduce heat to low and simmer for 10 minutes. Stir in clams and simmer until heated through, about 2 minutes longer. Serve hot (with oyster crackers, if desired).