Muddle nectarine in a shaker. Add rum, lime, Cynar, and peach liqueur, then add ice. Shake, double-strain into a chilled cocktail glass.
Double strain necessary to catch all that muddled nectarine! These recipes with muddled fruit work much better with a Boston Shaker and a Hawthorne strainer, with a wide spring, and a mesh strainer below as you pour into the glass. A capped cocktail strainer will definitely clog.