When you add flour to warm stock, it right away becomes clumpy. This happens because the molecules in the flower form a protective coating when they come in touch with the warm liquid. To avoid this clumping, start by melting butter in a saucepan. Then add the flower and stir until the mix is smooth. Lastly add your stock and voila no clump gravy!
The key is the butter that prevents the molecules of the flower to bind into clumps.