MELT 1/4 cup butter in large skillet. Add mushrooms, zucchini and onion; cook, stirring occasionally, for 5 minutes or until mushrooms and onion are tender. Add tomatoes; cook for 2 minutes. Transfer to large bowl.
ADD pasta, cheese, cream, reserved clam juice and remaining 1/4 cup butter to skillet. Cook over medium heat just until cheese is melted.
TOSS vegetable mixture with clams and pasta mixture. Season with salt and ground black pepper.