The Minister of Defence was the head of the Ministry of Defence.
In the 2000s, the office was held by the Saxon Master as "Harold Saxon". As Minister of Defence, he ordered the destruction of the Racnoss Webstar and designed the UNIT aircraft carrier Valiant. (TV: The Sound of Drums)
In 2018, the office was due to be taken up by Alexander Truscott. He planned to promote Wonderland in this position as part of a plan to build a new British Empire in space, but it was unknown if he ever took up office after his plans were ruined by the Twelfth Doctor. (COMIC: The Clockwise War)
Behind the scenes
In the real world, the Minister of Defence role existed in the UK from 1940 to 1964. In 1964, it was replaced by the Secretary of State for Defence.