CAIRO - One man was killed in clashes between security forces and supporters of ousted President Mohamed Morsi's Muslim Brotherhood on Tuesday, the first day of a constitutional referendum, security sources said.Mahmoud Sayed Gomaa, 25, was killed during a protest against the referendum near a polling station in the province of Bani Suef, south of Cairo.The referendum is the first ballot since the military overthrew Mursi on July 3 after mass protests against his rule.