[spoiler]Clarkson is from the Venom Incorporated universe. In their post-WW3 world, special forces agencies around the world are continuing to hunt for old Venom operators and destroy the extremely advanced weapons built by the company at this point. Clarkson, who was originally one of Venom's commanders, defected back to the SAS once Schrader had been killed, having been an operator with them prior to working for Venom. His expertise and knowledge on fighting the remaining scattered mercenaries is why he got promoted to being in command of the entire SAS.[/spoiler]