Put the cream and milk into a large pan and gently bring to just below boiling point. Meanwhile, in a large bowl, whisk together the yolks, cornflour, sugar, clementine zest and juice. Gradually pour on the hot milk mixture, whisking the sugar mixture constantly.
Wipe out the saucepan and pour the custard mixture back into it. Heat gently, stirring with a wooden spoon, until the custard is thickened, coating the spoon. Eat hot or cold.
You can make this up to 2 days ahead – just put a sheet of cling film directly onto the surface of the custard and chill, then gently reheat in a pan or microwave.