He wants to reply with a simple 'Of course!' without any hesitation, but promises mean more to Morgana than they do to the Doctor. If he makes one to her, he doesn't want to break it. He doesn't want there to be any reason in the universe to break it.
He keeps his concentration on her wound for just one more moment, then looks up to her.
no subject
He keeps his concentration on her wound for just one more moment, then looks up to her.
"What sort of a promise?" he asks, quietly.