For something to be a clause, it must contain a subject and a verb. An appositive phrase does contain a verb. It's a phrase, associated with a noun, which provides more information about that noun (note that in this sentence, "associated with a noun" is an appositive phrase).

Another example of a sentence with an appositive phrase is this:

"Johnny, my neighbor, planted a tree."

A similar sentence with an adjective clause is this:

"Johnny, who lives next door to me, planted a tree."