Born in Johannesburg and brought up near London, Clarke joined the Royal Artillery as an officer in 1916 but transferred to the Royal Flying Corps after finding he was too young to fight in France. He spent the First World War learning to fly, first in Reading and then Egypt. Clarke returned to the Royal Artillery in 1919 and had a varied career doing intelligence work in the Middle East. In 1936 he was posted to Palestine, where he helped organise the British repression of the 1936 Arab uprising. During the Second World War, Clarke joined John Dill's staff and proposed and helped implement, an idea for commando raids into France – an early form of the British Commandos.