STAMFORD — A Darien man has been arrested after police say he sold crack cocaine to an undercover officer several times. Frank Green, 34, of Allen O’Neill Drive, has been charged with three counts each of sale of narcotics and conspiracy to sell narcotics. Capt. Richard Conklin said the Stamford police department’s Narcotics and Organized Crime squad led the investigation with the help of the Darien Police Department.