Is it possible to smooth dirt/clay walls?
I know that you can smooth stone walls:
But my fortress is in a clay/dirt layer, and i would like to smooth the walls
You can't. Clay and dirt can't be improved at all, except by digging them out and building stone block walls in their place. For this reason, clay/dirt layers should generally be reserved for underground farms (since they don't require irrigation), and the real fortress should be built below them, in true dwarfish stone.