Ouch! That's a stone chip that has been allowed to go really quite rusty, and the paint has now started flaking off. You could use some rust converter-type stuff, but the finish will still be uneven, and you'd need to get more paint off the edges to reveal some unrusted bare metal, otherwise it will continue to creep underneath. Best thing is to get it done by a professional - smart repair or similar.
If only you'd caught it earlier on....
Easy to say, though