What you do is in the edit window, you select the text you want to hide and then there should be a little button that looks like a jagged line? You click that and it should ask you what you what the cut text to be, and you can just type something like 'Fic under here' or something and then click okay and then you can click save changes and the post should be updated.

Be sure to read the rules and FAQ, it should tell you how to do things like this.