Is the clause in parentheses an adjective or adverb clause? groucho marx, (whose real name was julius marx), was one of four famous brothers who were all comedians.

An adjective describes a noun, while an adverb describes a verb, and adjective and another adverb. To determine whether the clause is an adjective or adverb, we must know the object it describes. That would be Groucho Marx, which is a noun. Thus, the clause is an adjective.