The 25-year-old held talks with Clarke two weeks ago and is understood to be keen on a move nearer to his native Scotland.
Clarke said: "Graham came to see me a couple of weeks ago and he said it (wanting to leave) was down not to because of what is happening this season but previous seasons.
"He feels as though he needs a fresh challenge. We had an amicable discussion.
"We came to an agreement that, if something came along in January that was good for the club and good for Graham, we would listen to the possibility that Graham could leave."