Part of a play with the opening scene

ACT ONE

This division often follows an intermission and is labeled with Roman numerals; think of Shakespeare’s works where it marks the first major section.

This clue points to the first major section of a stage play, where the opening scene takes place. "Act One" is the traditional term for the initial act, setting up the story before later acts unfold. The phrasing plays on the idea that the "opening scene" is literally part of "Act One," making it a straightforward definition.