Will…I am not sure why your row action ‘as-is’ does not work. I was able to do this declaratively by creating a second model that I do not load on page load nor create a new row for. This is an empty model (you’ll see Skuid reference this as a ‘proto’ model). I selected the Id field that I want to update (in my case AccountId). In my row action I set the value of the lookup field to the Id from my ‘empty’ model. Since I have not loaded any data into the model, this Id field is empty/null. It does clear the lookup field. Here are some screen shots.

Pat Vachon has also pioneered a solution here.  Create a UI only field called “Blank”  its default value should be blank.  Then pass the value of this field into your lookup field in your action framework step.   

https://community.skuid.com/t/cannot-empty-the-value-of-a-lookup-through-the-action-fr…

Nice!  Thanks!