Final answer:
The adverb clause in the given sentence is 'because he called off the party,' which explains the reason for the subject not showing up.
Step-by-step explanation:
The adverb clause in the sentence "He hasn't shown up here because he called off the party" is "because he called off the party." This clause modifies the verb phrase "hasn't shown up here," explaining the reason why he hasn't shown up. An adverb clause is a group of words that functions as an adverb. In this case, it is providing the cause or reason for the main clause's action.