Smashcourts! I wouldn't call it clay. Clay is darker and harder and has higher bounces. Awesome facility overall as well, where you get an access pass for everything if you just buy the court fee (wich is just at 250thb per hour, no matter if it's day or night). Can use showers, fitness, etc.
I don't want to be too critical, but there is a little bit too much sand on it. It also doesn't help that nobody ever uses a mat to make the sand even level, normally you should do this on this type of surfaces. You can see there are small sandpiles here and there. But not terrible; you might get 4-5 weird bounces in total.
You can buy drinks.
Bit far away, but if you are a die hard tennis player it shouldn't stop you to go there. Besides that, if you're traveling with plane you might as well combine it with hitting some balls as it is close to Suvarnabhumi Airport. Excellent choise if you're flight is canceled or postponed.