Heat the oil in a large enameled cast-iron casserole. Add the onion; cook over moderate heat until softened, 5 minutes. Add the bell peppers and garlic and cook, stirring, for 3 minutes. Stir in the wine, water, fennel and crushed pepper. Add the clams and zucchini, cover and cook over high heat, stirring occasionally, until the clams open, 7 minutes.
Using a slotted spoon, transfer the clams to shallow bowls. Season the clam broth with salt and pepper and spoon over the clams. Top with the basil.