In a covered pan over high heat, begin heating enough water to cook spaghetti. Meanwhile, cook bacon in a 10- to 12-inch frying pan over medium-high heat, stirring often. Add garlic after 5 minutes; cook until bacon has browned, 8 to 10 minutes. Drain all but 2 tablespoons of fat from the pan.
When water is boiling, add spaghetti; cook, uncovered, until tender to bite, about 10 minutes. Meanwhile, to pan with bacon, add broth, clams with their juice, and olives. Bring bacon mixture to a boil over high heat; cover and keep warm.
Drain pasta well; toss with bacon mixture and tuna. Arrange on a serving platter or individual plates. Garnish with lemon and parsley.