Place the basil leaves in a cocktail shaker and muddle well.
Juice ½ grapefruit and pour in the shaker with basil leaves.
Pour rum, lime juice, and agave syrup into the shaker. Fill with ice and shake well. Pour into a glass.
Top with club soda. Garnish with a basil leaf and a grapefruit wedge or slice.
Notes
Taste after 1 ounce to see how you like it. You can always add more rum, but you can't take it away.If you're in a hurry, you can use bottled grapefruit juice.Grapefruit can interfere with certain medications, so you may want to investigate if yours does.