Vice President Dick Cheney, who returned to Washington last week from an around-the-world trip, is being treated for a blood clot in his left leg, his office announced today. Such clots, while rare, are sometimes caused by extended air travel. Cheney is being treated with a blood thinner intended to prevent the clot from breaking loose and reaching his lungs, said his deputy press secretary, Megan McGinn. She said Cheney, 66, had experienced "mild calf discomfort."