Is the clause who were all comedians an adjective clause?

Yes, the clause "who were all comedians" is an adjective clause. Here's why:

* Adjective clauses modify nouns or pronouns. In this case, the clause modifies the noun "they" (implied).

* They answer the question "which ones?" It tells us specifically which "they" are being referred to - the ones who were all comedians.

Example:

"They were all hilarious, who were all comedians."

The adjective clause clarifies which "they" are being discussed. It functions similarly to an adjective, adding descriptive information.