An adjective is a word that describes or modifies a noun. A clause is a group of words that has both a subject and a verb. If the clause expresses a complete thought, then it is a complete sentence. If it doesn't, it is what we call a dependent clause, as it depends on the main clause of the sentence to form a complete thought. An adjective clause, then, is a group of words that has both a subject and a verb that modifies a noun in a sentence. Adjective clauses are dependent clauses.