5 week since breast reduction and I have a slight split! What do I do

As mentioned in the other posts, this is not an unusual location to have delayed healing.  You may continue to shower and dress this area with antibiotic ointment and gauze.  It should continue to improve and fully heal over several weeks.  Continue close follow up with your surgeon.

This is a classic location for wound healing issues. Good wound care under the direction of your plastic surgeon should achieve wound healing and an acceptable cosmetic result.