I assume you just want to display the upper levels of the classification tree. There isn't any way a expression could accomplish this since it does not have access to the classification tree. However, this is a easy (short) Python script assuming you could just display (schedule) the result in a custom property.
In the Python script;
Extract the element guid.
use - GetClasificationsofElements to get your element classification level then extract the level number in front.
Use. GetAllClassificationsInSystem to get the class tree.
Get the upper level name by using:
return class[your top level number].classificationItem.id +'. '+ return class[your top level]classification.name