The clock does not belong on that mantle. It is not the right shape or color.

But, if you have to use it, then hang art work and set the clock in front of it.

The clock is slightly under-scaled in relation to the mantle & fireplace. To give it better scale and sense of importance, put a 4 1/2" plinth (box) under it which extends about 1 1/2" beyond the bottom of the clock. The plinth could be made of anything from wood (painted or stained) to marble or granite. If you were to use marble or granite , I suggest the same material to cover the brick fireplace surround which is out of place with the style & material of the fireplace surround & mantle.

Something has to give. The mantel, brick and clock are not working together. Consider removing the mantel and extending the brick to the ceiling and onto the entire wall, then decide if you'd need a mantel to put the clock on.

Large off center art leaning and clock on other side?

Not a designer... you have very nice built -ins and ceiling detail. Formal feeling. I would agree that the fireplace brick throws things off. Idea above from Allspace with altering that would make it more cohesive and then I would elevate the clock with large picture to the side.

Have you tried placing the clock on the shelves to the side?

The photo has distorted the proportions of the fireplace and room, making everything look wide and squat. A better picture would help.
I always like to center a substantially large painting over a large fireplace. If you have a painting in a frame in a color similar to the clock, that would be my choice if the size and theme are right. The clock could be placed off-center below the painting with a couple of candlesticks with candles on the opposite side of the mantel. Don’t put more than three or four things on the mantel to keep from cluttering it.
The fireplace does not have a hearth. If you can burn wood in the fireplace, you could add one: even a sturdy slab of stone laid on the wood floor might work (I would add a layer of heavy felt to the underside of the stone). It should be thick enough to be able to hold a large man without breaking. I think a fireplace without a hearth looks artificial.