An adverbial clause is a dependent clause which functions as an adverb.
The dependent clause "because he called off the party" functions as an <u>adverb of reason</u>. It offers a reason for the main clause "He hasn’t shown up here". It is answering the question "Why?"
Such clauses usually begin with subordinating conjunctions <em>because, as, since. </em>As we see, this particular clause starts with <em>because.</em>