That concrete blob is the real enemy here, not the post itself. If I were you I'd dig around the top of that chunk a little with a pressure washer or even just a hose on full blast. Water breaks up clay way better than a shovel ever will, and once it's muddy down there you can work the post side to side until it breaks loose from the concrete. Another trick is to drill a few holes into the concrete itself near the post, then soak it. Sometimes it cracks apart on its own after a good freeze. If you've got a buddy with a truck, chain it up low and give it a slow steady pull instead of yanking. Slow wins with clay every time.