It doesn't need to be a formal charge of a crime, it could also be something like a sexual assault allegation.

The allegation must be specific, like "I was attacked on [date] at [place]", it can't be "I heard a rumor that Scott once did something that made someone uncomfortable". And the alleged behavior must actually be a felony if it were to be proven in court, it can't just be "a bad thing".

I'll be broad with what counts as a "mainstream news outlet", it just can't be "a blog post from someone with 18 readers", or infowars.

This is part of a group of markets on Scott Alexander with slightly different resolution criteria, made with the intention of figuring out which structure is best. For an overview and discussion, see here.