Where can I download the stock clock apk for LG G3?
A "hard reset" would not change the /system partition, which is where the .apk originates. Since you deleted the system app, I'm going to assume that you have root access. After installing the .apk, I recommend manually pushing it to /system/priv-app/ (command: adb push LGAlarmClock.apk /system/priv-app/, replacing LGAlarmClock.apk with the full path to the .apk. Alternatively, you can change it from a user app to a system app (via an app like Titanium Backup if you install it from PackageInstaller, but I highly recommend pushing it to /system/priv-app/ from adb. Here is a link to LGAlarmClock.apk, pulled from the latest stock lollipop 5.0 build on XDA.