Clustered = phone book (aka sorted by something). You can only have one of these.
Non-clustered = back of the book index. You can have multiple of these. Don’t have too many non-clustered indexes though. You don’t want your index to be thicker than the actual book.
A thing to try: since indexes have to rebuild with new data, union a historical table to a current table to take advantage of the historical index and the size/speed of the current table.