Nanos present extra challenges for anemones. Go with the more tolerant specimens that don't exceed 6" diameter: Flower/Rock anemones and the larger version of S. tapetum(Maxi Carpets). Unfortunately, clowns do not typically choose them as homes.
fwiw: Tomato clowns easily reach 4"+ and will be out growing a 16g pretty soon.
fwiw2: If you were absolutely commited to clown and anemone only in the tank, you could try a BTA. Be prepared to remove anemone clones and I would recommend a smaller clown species long term.