It wasn't just a one off or a fluke, he's always had ability. It seems to have 'clicked' for him at an older age. But he did slow down in the second half of the season, in terms of goals and physical condition - and I know there are team factors contributing to the goals. I agree that he's capable of getting into double figures again, but near 20 again seems a huge ask.
I'm sure we're trying to sign a new main striker and as it stands I'd expect Clarke to start about 25 games, and score about 10 goals. That along with his age is the sort of basis I'd use to determine his value.
If we did sell him we'd unquestionably need to sign 3 new strikers - which wouldn't be ideal.
I'm pretty sure he'd have no interest in the move though, even if offered a considerable pay rise.