Captain Clarke was part of the Air Force squadron which was scrambled to intercept a stealth missile that had been launched from Iowa by Robert Morrison and Sabir Ardakani, terrorists working for Habib Marwan. His call sign was Delta 2. In his F-18 Hornet jet fighter, Capt. Clarke located the stealth missile (which was armed with a nuclear warhead and flying over Los Angeles) on his radar. He fired at the missile, which exploded, and moments later confirmed that he'd had a direct hit.
Background information and notes
Although Bill Buchanan explicitly called up the Air Force to destroy the missile, the Air Force does not fly F/A-18 Hornets; only the Navy and Marines do.