3 rounds of max reps Bodyweight Front Squat. Rest as needed between rounds.

An awesome front squat is key to a similarly awesome great clean. The leg positioning is NOT the same as a back squat or that is often used for an air squat. The back angle changes significantly as well. Take a look here at this illustration from Mark Rippetoe's Starting Strength

And take a look at Kolkov (at least I think it's Kolkov) - clearly not a shins back, butt back position. If they were, there'd be no way for him to have a snowballs chance at making this lift. BTW -- he should be red lighted here, impressive as the load is. Can you find the foul?