Because her son Draco Malfoy has been assigned a task by Voldemort that is far above his ability to accomplish, Narcissa wants Snape to help him, and do it for him if it looks as though he will as though he will fail.
Draco Malfoy has been assigned a task by Voldemort. This was because her son. This Voldemort is far above his ability to accomplish, Narcissa wants Snape to help him, and do it for him. It looks. He will. He will fail.