Climber for this sunny dry spot

I agree it's too hot to plant now .
I also dig a hole and fill it with water to ensure moisture.
Clematis like to be planted as deep as possible.
I'm not sure that the area you describe is that good for a hybrid clematis .

I used to dig a hole wider and deeper than needed, then water it 3 times till it stopped draining quickly and often giving it another soak at night, do this a few days running, then fill with a half and half mix of good compost, some grit and the dug out soil. Worked well for me in a hot climate.

Its not wise to plant new at the moment. If you have to, dig a hole, put in compost and water in the hole. Then add the well watered plant and add more compost and mulch. Water thoroughly. Water daily till established. A good climber for these conditions is tracheospernum, jasmine.

It isn’t recommended to plant anything in the border when the weather is so hot and dry. Before doing so, you need to add lots of topsoil, incorporated with humus rich compost. Ideally, plant after there’s been at least a week of steady rain. There are many varieties of Clematis to choose from. They like to be planted deeply, as they like their roots to be cool.