At the end of 1917, he became director of special intelligence at the Ministry of Reconstruction after being recruited on behalf of Christopher Addison, who had an interest in propaganda. When Addison left to become president of the Local Government Board, Clarke followed him in 1919 and then joined the Ministry of Health when it was founded, with Addison as the Minister of Health, in 1919. After two months he left to become editor of the "Sheffield Independent" before rejoining the Ministry of Health six months later, in March 1920. he was then seconded to Dublin Castle in August 1920 to lead the British propaganda effort during the Irish War of Independence.