Squeeze the juice from the clementines or oranges then push the juice through a sieve. This should leave you with at least 600 ml (1 pint) of juice. Bring to the boil and boil until reduced by half; in fact, whatever quantity of juice you have, always reduce it by half. Mix the arrowroot with a little water to soften it, then add it to the reduced juice a little at a time, stirring continuously until you have a sauce that will lightly coat the back of a spoon. Allow the sauce barely to simmer for 3–4 minutes. Remove from the heat and leave to cool.
When the sauce is cold, it should be a rich orange colour and have a full taste. If you feel it needs to be a little sweeter, just stir in some icing sugar.