1/4 cup fresh flat-leaf parsley leaves, chopped, plus 1 tablespoon thinly sliced stems
Bring a medium pot of generously salted water to a boil. Cook pasta until al dente. Reserve 1/2 cup pasta water; drain.
Meanwhile, heat oil in a large pot over medium heat. Add garlic and red-pepper flakes and cook, stirring, 30 seconds. Add wine and bring to a simmer. Increase heat to medium-high and add clams. Cover and cook just until clams open, 4 to 5 minutes. Discard unopened clams.
Stir in butter and parsley leaves and stems. Add pasta and toss to coat, adding reserved pasta water 1 tablespoon at a time, until sauce is loose and creamy. Season with salt and pepper.