Is this an adverbial or a noun clause?

Question

Yet the days crept by, and there could be no doubt that Fluffy was still alive and well behind the locked door. (Harry Potter)

Do you call the that-clause as an adverbial clause, or a noun clause in apposition with the previous noun? If you call it an adverbial one, what would you say it modifies?

Answer 1

In one approach at least, you can simply say that the clause is the complement of doubt, just as you would say that it was the complement of doubt if the latter was a verb.

Notice that it is in some sense part of the argument structure of the noun doubt that it can select a clause. Or in other words, you can't readily apply the same structure to any old noun: *the book that the universe is large, *his tolerance that Mary is rude. There's a property specifically of the word doubt (and some other nouns) that "licenses" the clause.

The term apposition is usually reserved for a much more general structure where you're implying that one phrase "equates" to another, but where the first doesn't necessarily "select" or "license" the second as such.

Answer 2

It's a That-complement, i.e, a noun clause. But it's not in apposition; rather, it's the object complement of the verb doubt

• No one doubted that Fluffy was ...

Nouns that are derived from verbs often take the same kind of object complements as the verbs do, occasionally with a preposition. So we get

• He attempted to help her ~ his attempt to help her.
• He enjoys (running) decathlons ~ his enjoyment of (running) decathlons
• She doubts that Fluffy is alive ~ her doubt that Fluffy is alive