Bring a large saucepan of water to the boil for the pasta. Meanwhile, heat the olive oil in a saucepan. Peel the garlic and crush it into the oil, saute briefly, then add the canned tomatoes and their juice.
Drain the clams, reserving the liquid. Put the clams aside. Add half the liquid to the tomatoes, along with the wine; cook over moderate heat, stirring occasionally, for 20 minutes, or until the mixture is reduced to a thick sauce.
When the saucepan of water comes to the boil, add the spaghetti, some salt, and the remaining liquid from the clams, if you wish. Return to the boil and cook the spaghetti, uncovered, for 10–12 minutes, until al dente.
While the pasta is cooking, chop enough parsley, if using, to give about 2 tablespoons.
Stir the clams into the thickened tomato sauce and heat through gently, without allowing the sauce to boil. Season to taste with salt and black pepper.
Drain the spaghetti, transfer to a serving bowl or individual plates and spoon the clam sauce on top. Sprinkle with parsley, if using. It is not usual to serve grated cheese with seafood sauces for pasta.