If you use the complete and follow on button in Workbooks, you'll find that reporting on Activity descriptions can be untidy as the description column will contain the entire history, including the previously completed task, rather than just the latest added comment (click to enlarge).

Instead we can remove the excess description - as highlighted above - using the substring_index function in a calculated column.

The formula required is: 

SUBSTRING_INDEX( description,'---------- Previous',1)

This formula finds the string '---------- Previous' in the description field and returns whatever is to the left of it, in our example, the most recently added description.