Just scruff up the root ball by hand. Be sure to see that the bottom part of the clematis is in the shade.

Don't trim off any of the roots!
If it's a relatively young you can treat it as if it was a new plant, just purchased. Trim back any particularly straggly stems; thoroughly soak the pot, to help removal; tease out the outer layer of the rootball; plant in a properly prepared hole; water well over the summer.
An older, established plant would probably be better left until early spring before being disturbed.

Hi, welcome to GoY, agree with the above, but would add that you should plant a few inches lower in the ground than it is in the pot, not only will this keep the roots cooler, but also if you get any Clematis wilt, and it dies back, you should get new growth from below ground, Derek.