To make the cake, place the clementines in a small pan, cover with water and bring to the boil. Reduce the heat to a simmer and cook for 1 hr or until tender. Drain and cool.
Heat oven to 180C/fan 160C/gas 4. Butter a 20cm springform cake tin and line the base with a disc of buttered baking parchment. Cut the cooked clementines in half and remove any pips. Place in the bowl of a food processor and pulse until finely chopped but not puréed.
Combine the raisins, sultanas, currants, cherries and brandy in a bowl. Add the clementine pulp and mix well. Cream the
butter and sugar together until pale. Add the beaten eggs, a little at a time, mixing well between each addition. In another
bowl, combine the spices, ground almonds, polenta and baking powder. Fold into the creamed mixture along with the dried fruit and clementine pulp.
Spoon into the prepared tin and smooth the top. Bake on the middle shelf of the oven for 30 mins. Reduce the oven temperature to 160C/fan 140C/gas 3 and continue to cook for a further 40 mins. You may need to loosely cover the top of the cake with a sheet of baking parchment for the final 20 mins to prevent it browning too quickly. Cool in the tin for 30 mins before turning out onto a cooling rack.
To make the topping, slice the clementines to a 5mm thickness. Tip the sugar into a saucepan with 140ml water and cook over a low heat, stirring often, until the sugar has dissolved. Put the clementine slices in the pan and stir through. To keep the clementines submerged in the syrup, cut out a circle of greaseproof paper to fit into the pan and place over the fruit. Cook over a low heat for 1 hr until glossy and translucent. Remove and spread out over greaseproof paper to cool.
To serve, dust the whole cake with icing sugar, then arrange the clementine slices, overlapping, over the top of the cake.