I tend to think he's done.
There's no one in the league that would know him better than Peyton Manning. And if Manning, after signing with the Broncos, told Elway that he wanted Clark in Denver ASAP, then it would have happened.
The fact that Denver went after the other Indy TE tells me that Peyton knows that Clark is not healthy enough to be a productive receiver and advised Denver to pick up the other one instead (his name is escaping me).
If he is done, it was a helluva run and he's accomplished more than 95% of NFL players will ever accomplish in their careers.