Heat the oven to Gas Mark 4, 180°C, fan160°C.
Remove zest from 3 clementines, cut away remaining peel and pith with a sharp knife and slice to approx 1cm (½in), removing seeds.
Line a 20cm (8in) springform tin with nonstick baking paper and arrange clementine slices over the base.
Cream together the butter, sugar and zest until light and fluffy, add 3tbsp hot water and whisk vigorously. Add eggs and whisk again - mixture may split but don't worry. Add flour to sponge mix and fold in until smooth and evenly blended.
Spoon into prepared tin over sliced clementines, taking care not to move the slices. Bake for 30 minutes.
Remove and cool a little. Place a plate on top and turn the whole thing over. Release spring and remove cake tin and baking paper. Sprinkle with a little sugar. Serve hot or cold.
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