It's not ideal but it should work. Like any other plant, clematis spread their roots laterally so they will grow sideways as well as down as far as they can or need.

Digging a large hole for planting is only a reflection of the size of the root system a well established clem can generate, which can be pretty extensive, especially with the larger growing vines.

Thanks, gardengal. I got it in with no problem and figured out it was the trellis hitting rock, not the future plant roots. The trellis is in there pretty good I think.