There are several explaining dictionaries on Словари и энциклопедии на Академике - with the radio buttons underneath the search box you can restrict the search to such dictionaries (which is actually the default setting). I very much like this one: Толковый словарь Дмитриева because it uses a set of simple sentence structures for its explanations. I even bought that one as a book. It only covers relatively basic vocabulary though.

It needs to be mentioned that some of the others are very old and pretty outdated in parts.