Lay the clementines on a plate or a small baking sheet and transfer them to the freezer. Freeze the clementines for at least 30 minutes.
Add agave syrup, water, and orange bitters to a saucepan. Whisk and bring to a boil. Turn to a simmer. Simmer for 5-10 min. Transfer to a cup & let cool in refrigerator.
Add clementines, Aperol, and simple syrup to a blender. blend until smooth. Add the ice and champagne and blend again until smooth and frothy.
When slushie reaches desired consistency, pour into 4 glasses and top with a dash of Aperol, more champagne, and a clementine slice for garnish. Serve immediately.