Okay, think I figured it out while re-watching. He did it very quickly in the video, missed it the first time: in the routes.rb file, he modified the original line:
resources :users, controller: "clearance/users", only: [:create] do
and removed the controller reference, so it reads like:
resources :users, only: [:create] do
Once I fixed that it worked fine.
But still: can you post example code? Thanks