It is possible. Since Goal Tracking is Profile based, the key is to have a tracking of all domains into one Profile. See the How do I install the tracking code if my site spans multiple domains? entry in the Google Analytics Help for further instructions. After that your iframe contents will appear as usual PageViews in the reports. (For instance, if you used <iframe src="http://otherdomain.com/stuff"></iframe>
, you will find PageViews for ’/stuff’.)
Otherwise, I don't really know what you mean under ’stuff is clicked’. If it's an object in the iframe you want to track, you may generate a virtual PageView when visitor clicks on it:
pageTracker._trackPageview("/Stuff_clicked");