Atrasou comes from the verb atrasar, meaning 'to delay' or 'to be late'. It's a common word, but its central role in the song's title and narrative makes it particularly interesting here.
In the song, "O relógio atrasou na hora de partir" (The clock was late at the time of departure) sets the entire scene of missed opportunity and regret. It's the pivotal event that causes the singer to be left behind, making it a memorable and emotionally charged word in this context.