You can send a private reply in a topic, which only allows the responded user to view the message content.
Go into the topic you want to post the reply.
Click under the post to open the Private Post form.
Enter your message and send it as described in the Posting a reply with Rich text editor section.
Private posts will not be checked for approval when they are posted in a topic that has the 'Posts moderation' enabled. Private posts are displayed with the 'Post Private!' label.