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.
Xavi's "Flores" is all about falling in love, and falling fast. He's not shy about it, either. He tells a girl, "De ti, en verguiza yo me enamoré, lo admito" (I fell for you super fast, I admit it). He knows his feelings are intense, saying, "Sé que suena un poco loco, pero es lo que es" (I know it sounds a little crazy, but it is what it is).
To prove his point, he makes a huge, almost funny gesture: "Te compré todas las flores pa' que ningun güey / Tenga la chanza para quedar bien contigo" (I bought you all the flowers so no other guy / has a chance to get on your good side). He's basically trying to buy out the entire flower shop to stop any other potential suitors! He sees her as completely unique and is ready to spend whatever it takes on her, because in his eyes, "vales un chingo" (you're worth a whole lot). It's a bold, confident love song about being completely smitten from the very first moment.