I'm not to sure what the OP means by "play" either. But if he's talking about a wobbling cassette as suggested in other post's.

I have read about instances where a freewheel has fallen apart while riding, Its tiny ball bearings and perhaps other parts spewing out all over the roadway. While some wobbling of the free wheel's cassette is normal, Perhaps excessive wobbling might be an indication of a potential problem in that regard. I didn't know that was possible for the free wheel/cassette to just fall apart on the road until I read about it.

I was concerned because in the reviews of the cheap SunRun 7 speed cassette the bike shop installed on my bike had fallen apart on a customer. I'm going to pay more attention to its operation on my next ride.