Maybe Clarke was the original actress to play Maya Hansen, back when the character had a bigger role in the movie?
It’s also possible that Clarke was the original actress tapped to play Ellen Brandt, although even though she wasn’t nearly as famous back then, that seems like it would have been too small a role for her.
Looking at the version of Iron Man 3 released to the public, there were only two primary characters, Gwyneth Paltrow’s Pepper Potts and Rebecca Hall’s Maya Hansen, plus Stephanie Szostak getting some screen time as Extremis-powered henchwoman Ellen Brandt.