Just a clarification, "!important" is a declaration, not a tag.
An "!important" declaration is put together with the value of your property. Example:
p{line-height: 10pt !important;}
Notice that I'm using points instead of pixels (px) as units. It's of good practice, when dealing with text, to always use 'pt' or 'em' to define their size. Only use pixels (px) as units when you want to manipulate margins, paddings, heights, widths, etc. of elements other than text.
Now, you have to see if the line-height of the element you want to define is nested in another element. For instance, if you want to affect all the <p> tags inside the <li> tags, then you'd have to do something like this in order to affect such element:
ul li p{line-height: 10pt;}
Noticed that I didn't use the "!important" declaration because I'm being more 'specific' in my element definition.
If that doesn't work (which it should), then you'd want to add the "!important" declaration.
You'd want to check also if the element you want to affect (I'm assuming is the <p> tag) has a CSS class, for example:
<p class="cssClass">text</p>
If it does, then you'd need to add the line-height property to the cssClass class.
Hope this helps.