UPDATE: It seems that powering down and restarting your device fixes this problem.
I’ve been receiving reports that, in some cases, defining words using the built-in dictionary is causing Marvin to freeze on iOS 7.
This seems to be an iOS 7 issue and there’s little that can be done until Apple come up with an update.
To verify this, open Apple’s own Notes app, write something, select a word and define it. You should see exactly the same thing happening.