Steam clams and pry open. Drain off and measure liquid; there should be 1 cup. Put the carrots, potatoes, and celery into the broth, cover, and boil gently until tender. Then cut clams finely with sharp kitchen scissors, saute the clams and onions in the chicken fat (or butter) for 5 minutes. Blend flour evenly into this mixture. Add broth and vegetables gradually, stirring constantly to keep smooth. Add the clam broth, cream and seasonings. Reheat just to boiling and serve immediately. Makes about 8 cups.