Bring a large pot of salted water to a boil. Cook the fregola until al dente, 6 to 8 minutes. Drain.
Meanwhile, heat a large skillet over high heat. Add the clams and wine, cover and bring to a boil. Reduce the heat to a simmer, cover and cook, gently shaking the skillet occasionally, until the clams open, 6 to 8 minutes. Transfer the open clams with tongs to a medium bowl and discard any that do not open; reserve the liquid in the skillet.
Add the fregola to the skillet and cook, stirring occasionally, until tender and the sauce is thickened, about 2 minutes. Remove from the heat and keep warm.
In a blender, combine the parsley, garlic, walnuts, olive oil, lemon juice and 1/2 teaspoon salt; blend until smooth. Stir the pistou into the fregola and return the clams to the skillet. Sprinkle with the lemon zest and serve.