Is 'the better part of an hour' an adverbial phrase?

Yes, that's correct! In the sentence, 'the better part of an hour' functions as an adverbial phrase, providing more specific information about the duration or extent of time it took to finish the project. Adverbial phrases modify the verb in a sentence, describing how, when, or for how long an action occurred. In this case, 'the better part of an hour' modifies the verb 'took,' specifying the duration of time spent on the project.