Pongo, you're assuming a spotlight sun, where, I guess, the sun somehow doesn't send out light in every direction. How that happens and doesn't affect the apparent shape of the sun is another matter.
Anyway, if the sun's light does go out in every direction, we should still be able to see direct light from the sun when the bottoms of clouds are being illuminated. In the traditional explanation, it's the atmosphere (or atmolayer if you prefer) that's limiting the distance the light of the sun can reach. The problem with that is that the light being reflected off the clouds is traveling further than the light coming directly from the sun, so the sun should still be visible.