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.
Mexican singer Alejandro Fernández teams up with the band Morat for a breakup track that pulls no punches. The whole song is a blunt reality check for an ex who only realized what she had after losing it. As the lyrics call out, "Tú nunca estuviste conmigo / Y ahora que por fin te dije adiós / Hoy quieres quedarte conmigo" [You were never with me / And now that I finally said goodbye / Today you want to stay with me]. He points out how she was only ever in love with his voice while taking his presence for granted, leaving her to wait alone at night now that he has walked away.
There is a clever linguistic twist here too. The song leans into the popular Spanish proverb "un clavo saca otro clavo" (one nail drives out another, meaning a new lover helps you get over an old one). But he warns her that trying to replace him with rebound flings will backfire: "Porque con ese clavo sólo abrirás otra herida" [Because with that nail you will only open another wound]. Even though he dishes out some brutal honesty by telling her "en el mundo hay tantas mujeres" [there are so many women in the world], he admits at the end that walking away still hurts him just as much.