Carson has been catching Trump, it was only a matter of time until there was a poll showing Carson ahead (apart from the idiotic IDB/TIPP poll that was clearly wrong). It needs further verification from other polls to be certain that Carson is really in front rather than this poll favoring Carson.
If Carson fails in the next debate like he failed in the last debate then the whole dynamic might change, at least in the short term, again.
Carson left a sponge in a patients' brain once! He's not intelligent enough to perform well in debates. Donald will destroy him on stage. Carson's fictional lead will vanish. Trumpster out.
Trump did badly in the last debate as well. Both candidates recovered, Carson more so than your boy. Whether the trend of the debates only having a short term effect on the polls continues we will have to wait at see. This debate might have longer implications as the Fox debate follows reasonably quickly, if a candidate does badly in both of them. However a good performance in the second debate might all erase memories of a poor first debate.
I am not convinced that Carson is leading but I am convinced it is fairly close between the two of them.