The answer is it depends! First the easy part: When comparing CPU clockspeeds within the exact same processor family, clockspeed is a good guide to performance because a higher clockspeed means more instructions are completed per second.
But when comparing the CPU clockspeeds of different processor designs, it's generally apples-to-oranges. For two CPUs with the same clockspeed, a shorter pipeline has an advantage because it stays full more often. For two CPUs with the same pipeline depth but different clockspeeds, the higher clockspeed gives it an advantage. For two CPUs with different clockspeeds, it depends on the pipeline depth and other factors.
For more information and a quick overview of how CPUs work see this article.