Sent messages cannot be unsent or removed from the recipient's Messages. Keep in mind that depending on a person's notification settings, email notifications containing a message's full content may be delivered immediately after the message has been sent on Facebook.
If you are referring to "How do I delete an Individual message from a Conversation" -
Open the conversation.
Select Delete Messages... from the actions menu at the top of the conversation.
Check the individual messages you’d like to delete and click Delete Selected at the bottom of the page, or click Delete All to delete the whole conversation.
For further information log into "My Account" the Help Centre
Norumbria. That is not the question. I have sent messages to people. I would now like to clear my messages I sent which is still on my FB under Messages. All I want to do is clear my old sent messages. I think the first answer is correct, I just don't want to believe it so if anyone know a way of me being able to clean up, clear out my old sent messages.