Apple - 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 "$(getconf DARWIN_USER_CACHE_DIR)/com.apple.appstore"
Just wait it out. If you attempted a fix listed in another answer here, quit all applications, restart your computer, and when you boot back up, open the app store to the update page, click the update button and forget about it. It took mine about 20 minutes to actually start and move out of "waiting". I read reports that for some people it took about an hour for it to start.
I tried everything else before and was ready to do a full reinstall of xcode. Nothing worked but for me to just wait.
I solved the problem by moving Xcode from my Applications folder to the trash bin. I didn't have an option to cancel the update (all the buttons under Update in the App Store were grayed out for me).
Once I typed in my password to confirm the move, the App Store prompted me to either cancel the update, or delete the app. Opting to delete the app caused the update to start downloading immediately.