Saying that sativa high is clear means that your mind is not slow and mellow, (as it is when you smoke some indica), but sharp, faster to percieving and thinking... at least for me, the indica gives a feeling of being "cloudy"... its hard to explain... it seems like there is clouds into your head, that prevent you from concentrating... yet sativa ones gives me a sense of concentration, of sharpness, that (at least for me) reminds of a cloudless sunny day (dont ask me why... im stoned. :stoned... i hope it makes any sense, and you can at least picture why its called a clear high...