The notion that clematis require or prefer alkaline soil is just one of a number of myths associated with this vine. Clematis species are native to numerous locations across the temperate regions of the globe and soil conditions vary widely. Some species may have more strict requirements but in general, clematis are not very fussy about pH......they are quite happy in both slightly to moderately acidic soil (pH 6.0-6.5), neutral soil (7.0) or slightly alkaline soil (7.5). Ideally, I'd target somewhere close to neutral (6.5-7.0) -- just avoid extremes in either direction :-) The American Clematis Society doesn't even address the question of pH being an issue!

It is more important that the soil be loose and friable and with a fair amount of organic matter included. And with very good drainage.

FWIW, I have grown a large number of clematis (60+) in moderately acidic soil (pH 6.3) without any issues. I even used to grow them up and through large rhododendrons, recognized acid lovers. I have never applied lime nor do I fertilize very heavily. I mulch well with compost each season and when I had rabbits, used their manure liberally. That's it....no other amendments or fertilizers. Lush, robust plants and with a profusion of flowers.

I've got 60+ in my acid soil yard too without ever adding lime. We lime the lawn and that is it.