Heat a large saucepan over a high heat. Add the chorizo and fry until the natural oil has been released and the chorizo is starting to colour. Remove the chorizo from the pan, leaving behind its oil, and set aside.
Fry the coriander and fennel seeds in the chorizo oil for 1 minute, then add the onion and chilli and fry until the onion has softened but not coloured. Add the garlic and fry for a further minute. Pour in the wine and leave to bubble until just 1 tablespoon of liquid is left. Add the tomatoes and stock, bring the stew to the boil and return the chorizo to the pan. Tip in the clams, then cover and cook until the clams have opened, discarding any that don't.
Divide the stew between 4 bowls, garnish with a few basil leaves and serve with crusty bread.