If the term is used as in "that Hobbit's feet are covered in curly hair similar to that on their head is canon" then I would say it is definition 5 with the added fillip of that rhetorical device, whose name I do not remember, where a thing is referred to by the name of the thing of which it is a part, as in "Washington announced today that ..." where Washington refers to a spokesman for some branch of the federal government. On the other hand, it could just be a shortening of the adjective canonical, in which case it would still be from definition 5 extended to include things other than the body of written works of a brick and mortar author.