This Clasp was awarded for service with the 8th Army between 23 Oct 1942 and 12 May 1943. An '8' is worn on the ribbon in undress to denote this clasp.
This award is controversial because Eighth Army was created in October of 1941 and fought in Africa for a year before the award service requirement. The award dates from the start of the El Alamein battle that ultimately led to the German eviction from Africa.
Reportedly General Bernard Montgomery refused to allow Eighth Army soldiers who fought under his predecessor, General Auchinleck, from October of 1941, and even his first few months of service starting in August of 1942 to wear the award.