Put the sugar and clementine zest in a small pan and cover with 150ml (1/4 pt) water.
Bring to the boil, then simmer for 5 minutes, until the sugar has dissolved and the mixture has turned syrupy. Remove from the heat and leave to cool. Once cool, pass the fluid through a fine sieve to get rid of the zest bits.
Combine the clementine juice and lemon juice with the syrup and stir. Pour into an ice cream machine and churn following the manufacturer's insturctions. Alternatively, pour into a freezerproof container, seal with a lid, chill for 1 hour, then whisk every 30 minutes for 2 hours to break up the ice crystals. Freeze until firm.
Remove the sorbet from the freezer about 10 minutes before serving to soften a little. To serve, scoop the sorbet into glasses using an ice cream scoop and garnish with mint, if you like.