Martyn without a doubt should be no. 4 in the Test side. I reckon most pundits would be in agreeance with this statement.
However, he is "unavailable" so the next guy up should be either Darren Lehmann or Brad Hodge, if we are basing selection on performance records (for middle order batsmen).
Why not Clarke? He still averages just 40 in First Class cricket. Now certainly, that doesn't make him an awful player, but what it signifies is that he is more of a liability at the crease than probably 15 other batsmen in Australia.
All I need to see from him is to add a few runs to his average, because that would show that his consistency has dramatically jumped. I know he's very talented and that he does fantastic work in the ODIs and a place in the Test side is probably his birthright, indeed I enjoy watching him bat, but I don't believe in rewarding talent without the necessary effort.