Check with your fishmonger whether the clams have been cleaned. If you need to clean them yourself, put them into a bowl of cold water with a tablespoon of fine salt and leave them for 40 minutes, moving them around gently every 10 minutes. The salt water makes them expel any sand.
Put the olive oil into a high-sided pan and put on a medium heat. Add the garlic and cook for 1 minute, then add half the parsley and the wine. Cook until the wine evaporates, then add the Cayenne and the flour. Stir, then add the fish stock. Continue stirring and, when it starts to come together, add the drained clams. Put the lid on quickly, and shake the pan a little to encourage the clams to open – you want them to open quickly in order to keep as much juice as possible inside the clams. Season with salt and pepper, and finish with the remaining chopped parsley.