In order to Clear Conversation in Groups.
Log in to CometChat admin panel.
From left side menu, click on "Settings->General"
In "Groups Chat" section search for "Number of messages that are fetched when load earlier messages are clicked:" and make text box count to "0".
Click on "Update" Button and from the left menu, click on "Clear Cache".
Clear Browser Cache.
Now you can see that "Conversation In a Group are Cleared"
We believe the above-mentioned steps were helpful to clear Conversation in groups.