"A three-act farce" is an appositive. It is not a clause as it lacks a verb.
An appositive is a noun or noun phrase that renames another noun right beside it. The appositive can be a short or long combination of words. The appositive tends to be placed between commas. In this particular case, "A three-act farce" is renaming the noun mentioned before: the PLAY. And it is separated from the rest of the sentence with commas.
