Step 4: Joining the pieces
Join the top to the sides with biscuits, or whatever pleases you, and then the sides to the front. Clamp it.
While you wait for the glue to dry, measure the wide of the inside of the clock and cut a piece of thin plywood with that wide and long enough to cover the clock face hole and a bit more. Check if it fits and do a mark in the clock frame and in that piece to remember its position also mark the outside of the clock face hole in it.