It’s hard to argue with that example, which leads to just one conclusion: Sometimes “because” can create a noun clause, which means that “the reason is because” is grammatical.

For example, “clever” in “she is clever” is a description or an adjective. Using nominalisation, “clever” becomes “cleverness” which is a noun.

An essential relative clause provides necessary, defining information about the noun. On the other hand, non‐ essential relative clauses provide additional, non‐necessary information about the noun.

In this example, the relative clause is ‘who smells of slime’. It provides more information about the man. ... Relative clauses come directly after the noun they are referring to.

In this example, the relative clause is ‘who smells of slime’. It provides more information about the man. ... Relative clauses come directly after the noun they are referring to.