For the spiced syrup: In a small saucepan, combine all the syrup ingredients and bring to a boil. Cook until the sugar dissolves, 1 to 2 minutes, then remove from the heat and let cool completely. Strain into an airtight container and keep in the refrigerator for up to 2 weeks.
For the spiced clementine sour: In a cocktail shaker, combine the whiskey, clementine juice, spiced syrup, lemon juice and egg white. Shake for a few seconds until frothy, then add ice and shake until well chilled. Strain the cocktail into a chilled coupe. Garnish with lemon bitters, clementine zest and a dusting of cinnamon, then serve.