Complex sentence: The position of "Secretary of the Air Force" in the United States is held by a civilian, and is the commander of the Department of the Air Force, which is a subsidiary department of the Department of Defense.
Split into simpler sentences: The position of Secretary of the Air Force in the United States is a sub-departmental position held by a civilian. His main job function is as the commander of the Department of the Air Force, which is a subsidiary department of the Department of Defense.