Bring 150ml/5fl oz water and the sugar to the boil in a saucepan until the sugar has dissolved. Remove from the heat and leave the syrup to cool.
Pour the syrup into a large bowl. Add the rum and the clementines to the syrup. Cover and store in the fridge for a week before using.
For the upside-down cake
preheat the oven to 180C/160C Fan/Gas 4. Line a 23cm/9in cake tin with baking parchment.
In a large bowl beat together the brown sugar and 100g/3½oz of the butter until smooth. Spread in the base of the lined tin. Arrange the clementine halves on top. Beat the remaining butter together with the rest of the cake ingredients. Pour the batter over the clementines and bake for 35–40 minutes. Remove from the oven and set aside to cool for 10-15 minutes.
Put the marinated clementines into bowls serve with ice cream and a slice of upside-down cake.
