Club root disease affects only plants in the Brassicaceae family (cruciferous vegetables), so you can safely plant anything from other plant families, including alfalfa.
https://www.gardeningknowhow.com/edible/vegetables/vgen/cruciferous-vegetables.htm
Avoid growing any Brassicaceae in the infested soil, because the organism that causes the disease is though to survive in the soil for years (some say 10 to 20 years!). Here is more information about this disease:
http://vegetablemdonline.ppath.cornell.edu/factsheets/Crucifers_Clubroot.htm
According to the linked article, legumes like Alfalfa and a few other plants can also carry the disease, but it doesn't damage those plants' health.
Seeds can spread clubroot only if they are dirty when they arrive, but the most common methods of spread are by wind and by transport of soil.