Ok... depending on how bad the overspray is, you're going to have to decide how aggressive you need to get with the clay. Not all clay is the same. If the overspray is really bad and hard to get off, you might want to look in to Swissvax Paint Rubber. This is a fairly aggressive clay and you're probably going to need a good polishing after. If the overspray is not that bad, I'd recommend Adam's Clay Bar (Grey professional grade). It's a less aggressive clay and shouldn't swirl very badly (if at all). Either way, I HIGHLY recommend Adam's detail spray as a Lubricant.

I good way to check would be to choose an inconspicuous spot and see if the overspray comes off easily with your fingernail. If it does, use a mild clay.

It's also possible that if you have access to a foam cannon, that a good soap might take off a very light overspray. Just suds the car up in the shade, let it work for a minute or two and then clean with a lambswool mitt (DO NOT USE A MICROFIBER MIT!).

I wish you the best of luck. If you have any questions, let me know.