Clausen grew up in a teacher's family in Aabenraa in North Schleswig, an area which Denmark had ceded to Prussia, following the defeat in the Second Schleswig War in 1864. Although forming a majority of the population, the Danes here faced critical times, being under strong influence of German administration, culture and language. Danish culture and language were, however, kept alive, not least through the growth of a quite strong folk singing culture. Clausen went to German secondary school, but in 1923 when he graduated from high school, the 1920 referendum following the German defeat in World War I had given North Schleswig back to Denmark. The mixed Danish-German culture of his childhood would be an inspiration for Clausen's work for much of his life, starting with a small autobiographical pamphlet with descriptions of his school days.