Hi, I would plant it 2-3 inches deeper than what it is in the pot now. Don't give it a lot of fertilizer as you want it to harden off before the cold weather comes. I would give it a little bone meal and mulch it well. You can prune it back after the leaves die back, but it might be better to wait until spring the first year. If you have a cold winter with little snow cover the plant will die back from the top down, so more top growth may be beneficial. Keep it watered well this fall. Etoile Violet is a very pretty one and very carefree. I am sure you will love it. Good luck with your clematis.

I'm in z5 Illinois and I agree. Make sure your hole is large (12-18" if clay soil, less if loamy) and I wouldn't worry about any fertilizer at all right now. Just keep it watered. This is a great time to plant perennials like clematis in our zone, and you should prune it all the way to the ground in mid to late February or early March, before the leaf buds swell.