It's really not too bad.
You just need a bit of practice with sampling with the clone tool.
It's also good to use the other healing tools together.
You can use the patch tool to clone larger areas, then break it up a bit with the clone tool, then blend it with the healing brush.
You can set the mode for those tools too, which helps alot when parts are too dark for the surrounding area you can set the mode to lighten or darken.
Alot of times it's useful to just blot down the clone tool instead of using strokes, especially if you're using a mouse.
Anytime I do any cloning I usually find it necessary to go in with the normal brush tool to retouch certain parts.
Another thing with your image is that you didn't clone out the shadow created by the fence you cloned out.