Combine 1 cup flour, undissolved yeast, sugar and salt in a large bowl. Add very warm water and oil; mix until well blended, about 1 minute. Gradually add enough remaining flour to make a soft dough. Dough should form a ball and will be slightly sticky. Knead** on a floured surface, adding additional flour if necessary, until smooth and elastic, about 4 minutes. (If using RapidRise Yeast, cover with a towel and let dough rest for 10 minutes before proceeding to next step.)
Pat dough with floured hands to fill greased pizza pan or baking sheet. OR roll dough on a floured counter to 12-inch circle; place in greased pizza pan or baking sheet. Form a rim by pinching the edge of the dough.
Combine alfredo sauce, celery salt and cayenne pepper in a small bowl. Spread over pizza crust. Top with clams, onion, olives, mozzarella and parmesan cheese.
Bake on lowest oven rack for 12 to 15 minutes, until cheese is bubbly and crust is browned.
Remove pizza from oven; sprinkle with parsley.
*If you don't have a thermometer, water should feel very warm to the touch.
**To knead the dough, add just enough flour to the dough and your hands to keep the dough from sticking. Flatten dough and fold it toward you.Using the heels of your hands, push the dough away with a rolling motion. Rotate dough a quarter turn and repeat the "fold, push and turn" steps. Keep kneading dough until it is smooth and elastic. Use a little more flour if dough becomes too sticky, always working the flour into the ball of dough.