1 tsp (or more) fennel pollen or 1 tsp fennel seeds, toasted and finely ground in a spice mill
Bring a large pot of salted water to a boil. Add pasta and cook, stirring occasionally, until almost al dente. Drain, reserving 1 cup pasta cooking liquid.
Meanwhile, heat 3 tablespoons oil in another large heavy pot over medium heat. Add chopped fennel and garlic; cook, stirring often, until softened but not browned, 2-3 minutes. Add wine, then clams. Increase heat to high. Cover and cook until clams open, 4-7 minutes, depending on size of clams. As clams open, use tongs to transfer them to a large bowl (discard any that do not open).
Add 1/2 cup reserved pasta cooking liquid to clam juices in pot; bring to a boil. Add pasta to pot. Cook over high heat, tossing constantly, until pasta is al dente and has soaked up some of the sauce, 2-3 minutes. Season with salt and pepper. Add clams with any juices from bowl and 2 tablespoons fennel fronds; toss to combine, adding more pasta liquid by tablespoonfuls if dry.
Divide among bowls. Drizzle with oil; garnish with fennel fronds and fennel pollen.