Charis,
How do you know that the soil is poorly draining?
When you water it does the water go down within a half hour? Or does it puddle and stay wet and not go down into the ground?
Time it.
Clematis doesn't need to watered religiously. How often are you watering it?
A couple times a week for a newly planted one is fine.
Especailly since it is hot.
If your soil puddles and the water just sits there after you water the plant, you must find a place that has better drainage. You could improve drainage, but its a lot of work and the clematis roots are going to be really large, huge, so I'm afraid just a foot of soil isn't nearly enough. Besides, if you have orange clay type soil, you will have a bowl effect and the water will still damage the plant.
Find another spot for it.

Is the plant yellowing? If not, you maybe just are not being patient enough. Clematis is truly one of those plants that fit the stereotype of first year sleeps, second year creeps, third year leaps. During the first year it will be putting on root growth. After you've had a hard freeze late in the year and any time until early spring cut the vine back to a couple of buds per stem. The second year it will be a bit more full, but still a fairly whimpy plant. During its third year it will really start to look like something and it will only continue to get better every year after that.