Clegg was captain of the nuclear submarine USS Jefferson.
As the Jefferson lost control, a giant doorway open up in a glacier. Suddenly, the submarine violently rocked, knocking him into the waters near Antarctica. When Clegg came ashore, he discovered the Jefferson. Exposed to freezing temperatures, he headed to Antarctic Weather Research Station 12 to tell the Third Doctor and Lieutenant Davis about the "doors in the ice" before dying. (COMIC: *Sub Zero)