"Is this a good idea or a beginner's mistake? And if it is a good idea, would you advise telling the publication that this is a chapter from an unpublished novel?"
And I don't think there's anything wrong with telling letting the press know that it's from a larger work when you submit the chapter.
<b>Asking our community: Have you published an excerpt from your novel? Is it a good idea to let the editors know that it's part of a novel when you submit? What kind of language did you use in your cover letter, and how did they respond? Let us know in the comments section.
Read more about how to get published, learn about various literary journals, or peruse other reader questions in these pages.

