In actual fact, this is not an uncommon injury to occur after a difficult vaginal birth, although it is obviously very distressing as a parent.

This site has some more information: Fractured clavicle in the newborn: MedlinePlus Medical Encyclopedia

In essence, there is no treatment necessary other than lifting the baby very gently in that side, it has no complications, and as the article says, often the injury is impossible to detect in later life, even with an x-ray.

Good luck. Lots of breastfeeding will help with the pain, and before long your little one will be as right as rain.