If you want to put clickable link at youtube description, you should not use HTML tag to make a link. To make URL clickable just put full URL strating with http://, example- http://www.google.com. Don’t start link with www. Never try to put html tag, it will not work. If your URL is long to put at youtube description, use tinyurl.com to make your URL short. Other important information is that when you put your URL at description, it will not become clickable just after editing, wait for atleast 7 hour after putting URL, after seven hour your URL will be clickable automatically. Once again i would say put your full URL starting with http:// to make your link clickable. Putting clickable link at youtube may be good for very high traffic at your website if your youtube video is popular.