Single Line Spacing
To get around this you can do what's called a "soft return" or "line break" by holding down the shift key while you press return. This puts the cursor to the next line but keeps it in the same paragraph (in html it's like entering <br> ... in fact you can see it if you click the HTML button in the Rich Text Editor if it's available). This trick works in most word processing programs, too.
Hope that helps!
Try this ..
Go to Themes from the Dashboard and look for the current theme
Select Customize theme
Select Add your CSS
Delete "1.8em" as the value for the line-height and insert 120%
Change this value to suit
ALso goto Typography.css in the themes folder and change the same thing and re-upload the file.
That's what I did after not finding any answers on here.
It got rid of a default 2 line spacing
Don't know if that's the right way to do it as I am a complete newbie to C5