He was a member of the Territorial Army at the outbreak of the First World War serving in the 5 Battalion (The Sutherland and Caithness) Seaforth Highlanders.
The 5 Seaforths left Thurso for Golspie on the 6 of August 1914, two days after the outbreak of war; Angus Clarke was in ‘A’ Company. The Seaforths were sent to France with the 152 Brigade, 51 (Highland) Division in early 1915.
Angus Clarke was then transferred to the 2 Battalion Seaforth Highlanders serving in the 10 Brigade, 4 Division at Ypres in Belgium. He joined the 2 Battalion as a battlefield replacement for the heavy casualties they had suffered in holding enemy attacks in the previous months.
On the 25 of April the 2 Seaforths attacked German strong points at the village of St Julien near Ypres and taking heavy casualties; at this spot today lies Seaforth Cemetery, Cheddar Villa containing the graves of some of the men killed that day. Total Seaforth casualties were 61 men killed, 252 wounded and 16 missing.