For promoting Clickbank or any affiliate products for free you should first have to have a good quality promotional page on your website for that product.
There are any number of free outlets you can advertise on for free.
YouTube videos can be made promoting that niche area with a link in the description page linking you your offer page.
Blogging about your niche program offer on the social networks will get visitors to your offer page. You’ll not want to overly hype the affiliate, just give them enough curiosity to visit your affiliate promotion page.
You’ll get traffic with interactive content on social sites, blogging on other blogs pulls in traffic. Social media posts and links, question and answer posts in forums, blog commenting, guest posting, classified ad links, image sharing, article and content submissions are the means to pull in traffic for free.