Keira Knightley has a way with costumes. She rocked the seven seas in “Pirates of the Caribbean,” picked up a bow for “King Arthur” and now she’s going full-on Victorian for “The Duchess,” which sees the star strapping on a corset for a scandalous royal love triangle as the Duchess of Devonshire. Below, check out an exclusive clip from the film which finds her fallen for a young politician. “The Duchess” — co-starring Ralph Fiennes — rolls out in limited release September 19.