Can you take a photo of it? Or do you know what variety it is or what time of year it normally blooms? Depending on the variety and how far sprouted it is, you may lose flowers if you trim it now. It won't hurt the plant in the long run, but it may reduce this year's flowers, though some do best with annual pruning and now would still be OK, though not for too much longer. If the forming buds are flower buds, I would wait until after it blooms and then prune it so that you don't lose the flowers.

IME in general, clematis will grow to the size they want to be each year, even when hard pruned. You can give it a shorter support to see if it will flop over and not climb as high. If you want a shorter clematis, it might be easier to move this one in the fall to a spot it can reach its preferred size and buy one that has genetics that will make it shorter.

If it dies back to the ground and grows up from there to the eaves every year you aren't going to be able to make it shorter.

IF it has bare stems for half it's height and grows up from there, then yes, whack it back to the ground.