Here are two ways to build an analog clock
https://groups.google.com/forum/#!msg/mitappinventortest/0knJibIeZIU/NxaqJGPUDwAJ
This is App Inventor 2. It cannot build service apps required to have an app run continuously or in the background.
To get the clock to display continuously full screen, you have to keep the device screen from sleeping and may run your battery down. You would use a stay awake routine using either an extension or blocks (search for stay awake in this Forum).
Regards,
Steve