OS X Yosemite Xcode update stuck on 'Waiting'
Yeah, a lot of people have been facing this issue. First, cancel your update. (In the AppStore update window hold Option to show the Cancel button.)
Now, you can do two things:

Download and install the Xcode DMG from here (if you have an Apple Developer Account)

Or try clearing your cache, and restart the download from the app store. You can use third-party apps or enter this command in your terminal:
rm -rf &quot;$(getconf DARWIN_USER_CACHE_DIR)/com.apple.appstore&quot;