UNICOI — A longstanding mom-and-pop Unicoi diner was destroyed by fire overnight. Clarence’s Drive-In, 4021 Unicoi Drive, caught fire late Tuesday night and quickly was engulfed in flames. The Unicoi County Sheriff’s Office and the state Fire Marshal’s Office were investigating. The restaurant was closed at the time. The diner was established from the former Engle’s Restaurant in the late 1960s.