1. Cook spaghetti according to package directions. Reserve 1/2 cup of the cooking water before draining. Drain pasta and set aside.
2. Melt butter in a large skillet over medium heat. Add garlic and cook 1 minute.
3. Add clams, crushed red pepper, lemon juice, and 1/2 cup cooking water; stir gently to combine. Cover and simmer until clams open and release their juices, about 6 minutes. Use tongs to transfer clams to a bowl.
4. Add cooked pasta and parmesan to the skillet with sauce. Cook, tossing, 3 minutes or until slightly thickened. Divide into 4 shallow bowls and serve.