It's not the best of circumstances since that clump of yeast will not rehydrate as quickly as it should. The best way I've found to deal with yeast clumps during rehydration is to swirl the hydration vessel (I use a stoppered erlenmeyer chemistry flask, but a glass jar is also fine) a couple of times during the rehydration interval to ensure that all the yeast gets equally dispersed. I will typically swirl once just after I add the yeast, and again after about 5 minutes have elapsed, to maximize the amount of yeast that gets suspended in the liquid.