I don't think it necessary to make clay a resource.
A good? Yes. However we work out that goods need to be expressed. I still favor them as non-map generated resources.
But how about this... I like the idea of making a special sort of improvement that can be built only on freshwater access tiles (water being important so as to make the processing and extraction possible) called a Clay Quarry.
The Clay Quarry would give +1

and from that somehow derive a uniquely available building that creates Good (Clay) in the city provided that the Clay Quarry was within the workable boundaries of the City Radius.
Perhaps could grow in effectiveness, but shouldn't be quite as effective as a workshop in full force (though perhaps a bit better in the early stages as it wouldn't reduce food.) Probably should NOT be buildable on Tundra or Peaks. Limited to freshwater is about right. Will make for some very tough choices in some games on how to work your capital. Interestingly, with proper irrigation techniques it could be put in a string of farms maintaining an irrigation line while adding a bit o' production instead of food. Though this wouldn't be best as something you'd want to build everywhere.