Coarsely chop and puree the whole orange in a food processor. Transfer to a bowl and add melted butter, eggs, mixed spice and sugar. Mix well.
Sift the flour into the mixture and beat it well. Pour mixture into a greased 18cm loaf pan.
Bake at 180°C for 40 minutes. To test whether your cake is ready, insert a wooden skewer into the centre – if it comes out clean, the cake is baked. Remove from the oven and transfer to a rack to cool.
To make the icing, combine the orange juice one tablespoon at a time with 1 cup of icing sugar and mix until it is a thick pouring consistency. You may not need all the juice. Pour generously over the cooled cake