	Clark has specialised in Commercial Property Law and written extensively in the law of Landlord and Tenant and Commonhold. He was responsible for the Boundaries and Commonhold titles in Halsbury's Laws of England. Clark was a member of the Department of Constitutional Affairs Commonhold Consultation Group. He was also part a group advising the Law Commission on the reform of the law of easements and covenants. He is a co-editor of the "Common Law World Review".