A secret compartment that is hidden in plain sight just don't let others change the battery.
Step 1: Getting In
Take off the back plate that covers the battery and mechanism
Step 2: Stashing the Loot
Now take what ever you want to hade from others and put it inside the clock. If you prefer you can even build a compartment or box to put in the clock.
Step 3: Hide
Now replace the back plate and put up in the mantel or shelf.