That's the neat part about clean energy, it doesn't mean anything. It's a term made up by the energy industry to compete with the idea of green energy so that people conflate the two. What is meant by clean energy in this context is that it doesn't produce unwanted products from side reactions. It just makes CO2 and H2O unlike say coal which can produce SOx and NOx or Bunker fuel used in container ships which creates all sorts of nasty shit when it burns. Green energy on the other hand means that it produces very little if any CO2.