Usually When using expandable text boxes there isn't a way to clear them unless having a reset button underneath which won't work very well when displaying information in a grid.
If you must apply this feature, consider de-prioritizing and protect the user. A familiar area to show additional input information is the bottom right corner often used to display a character count. A simple text link might suffice. Additionally, offer a way for the user to undo this action to prevent mistakes.
Unless this feature is specifically beneficial to your use case, one reason to reconsider is: Long inputs call for a larger investment to write the input. It is unlikely for users to forgo all the invested time and energy on one click. If done by mistake, this could lead to a lot of frustration. Protecting user data is very important in form design.
More about the topic - https://www.nngroup.com/articles/reset-and-cancel-buttons/