Clear is a procedure used to clear the values stored in both the Headers and Body of the message. 

Clear calls ClearHeader to remove the contents of the message header. 

Clear calls ClearBody to remove the contents of the message body including any text or attachments founbd in the MessageParts collection. 

Clear is called in LoadFromStream to reset the content of the message prior to loading values from the stream. 

When the message is used with TIdNNTP, Clear is called prior to retrieving the newsgroup article or headers. 

When the message is used with TIdPOP3, Clear is called prior to retrieving the headers or body content for the email message.

TIdMessage.ClearHeader 

TIdMessage.ClearBody