If you have an ice cream freezer, this recipe works equally well as a sorbet.
In a small saucepan, stir together the sugar, zest, a pinch of salt, and 3/4 cup water. Bring to a boil over medium heat and cook, stirring, until the sugar dissolves and the syrup is clear, about 2 minutes. Set aside to cool slightly as you juice the clementines.
Stir the juice and syrup together, pour into a small metal pan, such as a loaf pan, cover with plastic, and freeze for 2 hours. Stir the mixture with a spoon, breaking up the portions that have become solid, and return to the freezer. Stir every 30 minutes until the mixture is evenly icy and granular, about 2 hours more.
Cover and return to the freezer until ready to serve. To serve, scrape with a spoon to loosen the mixture, and spoon into small bowls or glasses.
The granita may be made up to one week ahead.