It's a mystery where he works. The underlined clause is a/an:

The underlined clause "where he works" is an adjective clause.

Here's why:

• An adjective clause modifies a noun or pronoun. In this sentence, it modifies the noun "mystery".

• The clause answers the question "What kind of mystery?"

• Adjective clauses often begin with relative pronouns like "who", "which", and "where".

Therefore, "where he works" is an adjective clause that provides additional information about the mystery.