Clavar literally means "to nail" or "to hammer something in". While it's a common verb, its slang meaning is what makes it special and perfect for a love song.
In this context, the phrase clavarse con alguien means to get completely hooked on or to fall hard for someone. When Xavi sings "¿quién no se va a clavar?", he's asking rhetorically, "who wouldn't fall hard for you?", expressing a powerful, all-consuming infatuation.
In "Flores", Mexican singer Xavi doesn't waste any time playing it cool. He admits straight away that he fell head over heels at lightning speed, using the Mexican slang phrase "en verguiza yo me enamoré" ("I fell in love super fast"). His game plan to win her over is hilariously extreme: he buys up every single flower available ("Te compré todas las flores") just so no other guy ("ningún güey") even gets the chance ("chanza") to look good in front of her. He knows it sounds wild, but he is completely unapologetic about how hooked he is.
The whole track rides on this bold, lavish energy. He is ready to drop serious cash ("me gasto un billetón") on whatever she wants because, in his words, "vales un chingo" ("you are worth a whole lot"). He describes her as his newest bad habit ("Ya te me hiciste vicio" [You became an addiction to me]) and asks how anyone could resist falling hard ("¿quién no se va a clavar?") for someone so pretty. It is an intense, direct confession packed with everyday Mexican slang that shows he is planting his feet and refusing to walk away.