I would reccomend seeking professional help to rectify this. If you want to try this yourself you could apply a patch (on the back) using a very thin piece of material and some flexible glue that is suitable for the job. This would help to strengthen the area and stop it getting any worse but you would still have a visible split. Depending on the type of leather the split could be repaired back to new.
Try http://www.leathercareandrepair.co.uk/designer_leather_product_repair.html
JC
January 2010