One sees the light every so often

The answer to the crossword question “One sees the light every so often is MOLE” is referring to the fact that moles are creatures that live underground and don’t typically come out into the open. However, every so often a mole may surface, allowing someone to catch a glimpse of them before they burrow back into the darkness. So, the clue is using a bit of wordplay to suggest that someone who “sees the light” every so often is like a mole, living a life that largely takes place in the dark and with brief, occasional glimpses of the world above.