Last March I posted an entry here named Google Definitions Help with Rankings and they did. I was ranking very well for very popular keyword phrases in the form of "what is keyword". I still do rank well for the same keyword phrases in that format, however, traffic is now a small fraction of what it was in the past.
The main link to the definition was a direct link to the first definition that came up. So if someone clicked on that main link, it would go directly to the page with the definition (i.e. my page). Now the main link goes to the "other definitions".
Old screen shot:
Now what you will see is that it reads "Web definitions for keyword" and then shows the first definition and then has an other smaller link to "Definition in context" which links to the first result. Traffic from Google definitions has dropped dramatically ever since this change has been put in place.
New screen shot: