Clavo literally means "nail", but it's used here as part of a famous Spanish proverb. The saying "un clavo saca otro clavo" is the equivalent of "the best way to get over someone is to find someone new".
In this song, Alejandro Fernández cleverly twists this idea. He sings, "Porque con ese clavo sólo abrirás otra herida" (Because with that nail you will only open another wound), warning his ex that a rebound relationship won't heal her, but will only cause more pain. It's a powerful metaphor that turns a common saying on its head.
This song is the ultimate breakup anthem for when you're feeling both powerful and a little bit sad. Mexican superstar Alejandro Fernández teams up with the band Morat to tell an ex-lover exactly how it is. He starts off strong, almost rubbing it in her face: "Sé que te duele / Que ya no quiera verte" (I know it hurts you / That I don't want to see you anymore). He claims she only wanted him after he was gone and that she was never really there for him in the first place, saying she only "te enamoraste de mi voz" (fell in love with my voice).
But just when you think he's completely moved on, the song reveals he's hurting too. He watches her try to move on with new people, warning her that "con ese clavo sólo abrirás otra herida" (with that nail you'll only open another wound), a play on the idea of a rebound relationship. In the end, despite all his tough talk, he admits that seeing her lost "también me duele" (also hurts me). He confesses that he still loves her, and that's the real pain behind the song.