Complex sentence: On one hand, if the word "heterological" "is" heterological, then it does not describe itself, so it is an autological word, which means it isn't heterological.
Split into simpler sentences: On one hand, if the word "heterological" "is" heterological, then it does not describe itself. Since the fact of it not describing itself does, in fact, describe it, it is autological, which means it isn't heterological.