Sorry, but that is not a clematis :-(( It looks very weedy.

I don't think that's your clematis. :( I looked at a bunch of images of them online and if you look at their leaves they are a completely different shape.

Attaching a picture I found from google that shows the leaves pretty well.

I agree that looks like a weed. Personally, I'd pull that, but wait to see if the clem comes back, even as late as next spring.

I would put a circle of low wire fencing, staked in place, to prevent a repeat of the mowing incident. Once/if the clematis returns (and it may be as late as next season, depending on how large the clematis was to begin with) then you need to put up pea netting, a trellis or wrap the post in wire fencing. Clematis climb by wrapping the leaf stems around a narrow support, so it doesn't matter how textured the post is since it is thicker than 1/2".

That is a weed, not your Clematis.

Excellent. I'll pull away! I'll end up replanting, as the clem was a bare root, maybe only a season old. I also didn't raise the mound enough, I think. Is it too late to plant a new species now, in late may/early june (zone 9)?

I would wait a year to replace it as it may still come back. Do as nabs suggests.with fencing the area and put down some alfalfa pellets and wait.
Clem's are amazingly resilient. Last year I had one small one not appear at all bit this spring it is growing.