It will take a bit of prising off, and expect there to be a cracking sound when it releases. I think it may have been glued together or something. As gary says, it's just a protective cap. As I recall, there is a plastic sort of tamper seal on the rear of the clock. You'll have to break this in order to get the back off. Don't worry about the seal. The screws will hold the back in place once you put the thing back together again.
Maybe take some pictures for the wiki whist you're in there...