Combine the sugar and 1 cup water in a medium saucepan and bring to a boil. Reduce the heat to simmer and continue to cook until all the sugar dissolves, 3 to 4 minutes. Take the pan off the heat and add the mint leaves. Allow this mixture to steep for 10 to 15 minutes and cool down. Discard the mint.
Fill a large pitcher with the sparkling water and clementine juice. Slowly add the cooled syrup to your desired sweetness and taste.