According to Wittgenstein, what we cannot speak about cannot be defined clearly (hence, cannot be defined at all). Because if it could be defined clearly, it could ipso facto be spoken about. What cannot be spoken about can only be shown, and even that only indirectly, by showing what can be spoken about.
The book will, therefore, draw a limit to thinking, or rather —not to thinking, but to the expression of thoughts; for, in order to draw a limit to thinking we should have to be able to thnk both sides of this limit (we should therefore have to be able to think what cannot be thought). The limit can, therefore, only be drawn in language and what lies on the other side of the limit will be simply nonsense. (Tractatus, Preface)
Concerning the other question, I'm not aware that anyone argued that the Tractatus implies that the world of facts cannot also be spoken about. Many have suggested the opposite, i.e. that the Tractatus itself shows that more can be spoken about than is supposed. Thus Bertrand Russell, in his Introduction to the Tractatus:
These difficulties suggest to my mind some such possibility as this: that every language has, as Mr Wittgenstein says, a structure concerning which in the language, nothing can be said, but that there may be another language dealing with the structure of the first language, and having itself a new structure, and that to this hierarchy of languages there may be no limit.