Have you read the documentation?
In your code (and I'm assuming you've seen or are using their sample code) there is probably a line something like this:
PHP Code:
foreach ($feed->get_items() as $item):
This is the top of a
foreach loop that retrieves a SimplePie_Item (one item in the RSS feed). Within that loop, each time it retrieves an item, you have access to methods like
PHP Code:
$item->get_description() (which gets the description)
$item->get_title() (which gets the title)
Methods are things you can "do" with that particular item.
What you need to do is read through the
API documentation for the item level methods for the SimplePie_Item and find the one that does what you want.

.
There are a lot of things you can "get" - maybe you'll find some other options you want to use while you're looking.