On 30 September, the "Gruppe" moved to an airfield at Uetersen, flying defensive missions over the German Bight. The unit relocated multiple times and was flying in defense of the Reich from Esbjerg on 10 May 1940, the start of the Battle of France. On 14 May, I.(Jagd) "Gruppe" transferred to Essen-Mülheim for operations against France. On 23 May, they transferred again, this time to Ferme Montecouvez, an airfield approximately south of Cambrai. Two days later, I.(Jagd) "Gruppe" was tasked with providing fighter escort for German transports resupplying the 4th Army in the vicinity of Cambrai. On this mission, Clausen claimed an "Armée de l'air" Potez 63 twin-engined aircraft shot down. In this encounter, his Messerschmitt Bf 109 E was also damaged, resulting in a forced landing near Cambrai. On 15 June, I.(Jagd) "Gruppe" was moved to Saint-Omer, the armistice was signed on 22 June, ending the Battle of France on 25 June. The unit was given three days of rest, some of the pilots were sent on home leave. On 30 June, I.(Jagd) "Gruppe" was scrambled to intercept a flight of Royal Air Force Bristol Blenheim bombers resulting in combat southwest of Saint-Omer. In this encounter, Clausen claimed a Blenheim from No. 110 Squadron shot down which was not confirmed but was himself shot down in his Bf 109 E. Clausen was awarded the Iron Cross 1st Class () on 4 July 1940 and was promoted to "Oberleutnant" (first lieutenant) on 1 February 1941. That day, Clausen was also appointed "Staffelkapitän" (squadron leader) of 1.(Jagd) "Staffel" of LG 2, this squadron became the 1. "Staffel" of "Jagdgeschwader" 77 (JG 77—77th Fighter Wing) on 6 January 1942.