Your clematis looks happy and healthy and this vine tends to grow flowers and leaves at the top unless you prune early each spring. The best time to prune is determined by when the vine flowers. I am not sure what variety you have by looking at the photo. Perhaps you can send a photo of the blooms or know what variety it is? The general rule of green thumb is to cut back summer blooming clematis to within two feet from the ground in early spring when you see signs of growth. Leave at least two sets of healthy buds on the vine. You can also let the vine go natural for a few years and just plant a lower growing shrub or perennial at the base of the arbor to hide those naked knees of your clematis. A small evergreen azalea or even a container garden on either side of your arch will help to shade the roots of your clematis vine. All clematis like their roots kept cool and their tops in the sun. Keep growing, Marianne Binetti