Put the sponge in a large bowl and sprinkle with the prosecco. Leave to stand for 5 minutes.
Meanwhile, whip the cream with the vanilla and icing sugar until it forms very soft peaks.
Put a layer of custard in a large trifle bowl. Follow this with a layer of sponge, then a layer of whipped cream. Scatter over a third of the clementine segments. Add another layer of custard, sponge, clementine segments and cream, making sure it completely covers the fruit.
Arrange the last of the clementine sections on top and scatter over the pistachios and the glitter sugar.