issement is a noun that refers to an enlightening explanation.

The French term eclaircir (ee CLAIR seer) means ‘to clear up’ or ‘enlighten.’ An eclaircissement is an act of clearing something up or enlightening them.

Robin’s eclaircissement of Mr. Harrington’s behavior helped me understand things much more clearly. I can now see exactly why he yelled at me every time I offered him scrambled eggs.