Just redirect a domain to your affiliate link. I've been doing it for a while with my cpa network and always had more success with doing that than linking the affiliate link in the description. I'm pretty sure they allow affiliate links in description though, I know some big youtube networks have affiliate links to their clickbank products in description and seen loads of people doing it.

I think you can include an affiliate link in the description, although it is frowned upon by Youtube. Masking the affiliate links with a redirect can be of help.

But a better strategy is to link from the video description to a lander and then from the lander link to the CPA offer/Clickbank product. That's for multiple reasons - You can do some preselling on the landing page to make the traffic convinced they want to purchase the product/fill out the offer, etc.

You can also link to squeeze pages, build a list along the way and then market to the subscribers on a continuous basis.