The “Clean Up Conversation” feature in Outlook allows an end user to delete redundant messages that have been replied to in a conversation or folder.
- Select a conversation or folder that you would like to clean up.
- On the “Home” tab, click the drop-down arrow next to the "Delete" button as shown in the picture below.

- A confirmation will pop-up by default. Confirm by clicking "Clean Up."
NOTE: You can check the "Don't show this message again" box and then click "Clean Up" to prevent this prompt from popping-up in the future.

This feature can resolve clutter in specific conversations, but you can also clean up entire folders by using "Clean Up Folder" instead.
Reference:
Clean Up Your Inbox - Microsoft Support