Professor Clarke's distinguished career in the defence academic field began with a degree in international politics from Aberystwyth University, where he subsequently took an MSc(Econ) in British Defence Policy and served as a researcher in the Department of International Politics. He moved to Manchester University from 1975 to 1979 and then to the Department of Politics at Newcastle where he remained until 1990 when he became the  founding Director of the Centre for Defence Studies at King's College London. He was appointed Professor of Defence Studies at King's in 1995 and became Director of Research Development and Deputy Vice Principal there in 2005.

Professor Clarke has been a senior Specialist Advisor to the House of Commons Defence Committee since 1997, having served previously with the House of Commons Foreign Affairs Committee. In 2004 he was appointed the UK member of the United Nations Secretary General's Advisory Board on Disarmament Matters. He has published extensively on defence and security issues and has  lectured at many universities in the United Kingdom, as well as at the Joint Staff College, the Royal College of Defence Studies, the NATO School at Oberammergau and the Clingendael Institute in the Netherlands. He has been a Guest Fellow at the Brookings Institution in Washington and a Fellow in British Foreign Policy at the Royal Institute of International Affairs, of whose Council he has been a member since 2004.