Bring the clam nectar to boil in a large pot. Add the clams; cover and cook just until they open. Transfer the clams to a large plate; strain and reserve the cooking liquid. When the clams are cool, remove the meat from the shells; discard the shells. Pour any liquid on the plate the clams sat on into the reserved cooking liquid. Melt the butter in a pot over medium heat. Add the onion, celery, carrot and garlic and cook until tender, about 5 minutes. Sprinkle in the flour and mix well. Slowly stir in the reserved clam cooking liquid. Add the potatoes, milk, tarragon and bay leaf and bring to a simmer. Simmer until the vegetables are just tender, about 10 minutes. Add the reserved clam meat and simmer a few minutes more. Season with salt and pepper, pour into bowls, sprinkle with chopped parsley, and serve.