Clerke was a native of Bandon, Cork. Being intended for the army, a profession also adopted by his brothers, St. John Augustus Clerke, who died a lieutenant-general and colonel 75th Foot, 17 January 1870 and William Clerke, afterwards a major in the 77th Foot, he was sent to the Royal Military College, Great Marlow, where he distinguished himself by his abilities. He was appointed to an ensigncy without purchase in 1808 and as a subaltern in 28th and 5th Foot, he served through the Peninsular campaigns until the loss of his right leg in the combat at Redinha in 1811 incapacitated him for further active service. On the recommendation of Lord Wellington, he was promoted to a company in the 1st Garrison Battalion, with which served until its reduction in 1814.