When one writes "X is a racist" (or "white nationalist", or something like this, or something that essentially means that), a reader would certainly assume that you're talking about modern and commonly known representatives of this class. If after that you have to qualify - no, I didn't mean the common meaning of that word but I meant some arcane and archaic meaning that is not immediately obvious but means something different that I could define if I was called on it - I think you own the misinterpretation. If you used the non-obvious meaning, you should explicitly state that, since you know there is an obvious meaning which would completely overwhelm any other meaning.