It's difficult if you are not comfortable with editing the HTML of your post. But if you want to have a go, here it is.
Type out your post as you normally would, including the content you want hidden. Click the HTML button to view the HTML of your post.
It's easier if the content you want hidden, is contained in one paragraph. Example below...
<p>I want this text to be hidden.</p>
To hide the paragraph, you need to modify the opening paragraph tag <p> by adding the following text. style="resize:vertical;overflow:hidden;height:5px". So it looks like this...
<p style="resize:vertical;overflow:hidden;height:5px">I want this text to be hidden.</p>
The height value of 5px can be adjusted depending on how much of the text you want to be seen before the box is expanded.
I have not included a maximum height in the code to make it as easy as I can for you to do, but this means the box can be expanded indefinitely. To prevent this, add the following code...
<p style="resize:vertical;overflow:hidden;height:5px;max-height:200px">
...the maximum height value, currently 200px, will need to be a value that suits the amount of content. Too lower a value and the box will not expand enough to allow the whole content to be seen, too higher a value will reveal nothing more than empty space, wasting the content readers time.
If your content covers more than one paragraph, as below...
<p>I want this text to be hidden.</p>
<p>I want this text to be hidden as well.</p>
...then you will need to create a <div> container for the two paragraphs and put the code in the opening <div> tag like this...
<div style="resize:vertical;overflow:hidden;height:5px">
<p>I want this text to be hidden.</p>
<p>I want this text to be hidden as well.</p>
</div>
That might seem a bit of a nightmare, but you won't break anything by having a go, and if you can't get it to work, you haven't lost anything, apart from some time. : )