Cut the clementine in half. Juice one half and finely chop the other. Pour the freshly squeezed juice into a serving glass and stir in the demerara sugar until dissolved.
Add the chopped clementine and chopped lemon wedge to the glass. Using a muddler or the end of a wooden spoon, gently press the fruit to release the juices and essential oils.
Fill the glass with ice, then scatter in the mint leaves and add a few drops of orange blossom water. Slowly top up with sparkling water and stir gently to combine. Serve immediately for a refreshing, citrusy mocktail.