Blend the mincemeat, clementine zest and juice in a food processor to a smooth paste.
Put the puff pastry on a well-floured surface with the short side towards you. Roll the pastry a little thinner and cut it in half widthways. Brush each piece of pastry with melted butter and sprinkle over the icing sugar. Spread the mincemeat paste over the puff pastry.
With the shortest end facing you, roll both long edges of one piece of pastry towards the centre. Brush a little beaten egg down the centre to stick the two halves together. Repeat with the remaining piece of pastry.
Carefully lift the rolls onto a baking tray and leave to chill in the fridge for 30 minutes.
Preheat the oven to 200C/400F/Gas 6.
Remove the roll from the fridge and, using a very sharp knife, slice it into 1cm/½in thick pieces. Lay each slice on the baking tray, cut-side up, and brush well with the beaten egg and a further dusting of icing sugar, then bake in the oven for 15 - 20 minutes, or until puffed up, crisp and golden-brown.
Remove the palmiers from the oven and leave to cool on the baking tray. Dust with more icing sugar before serving.