Its hard to say for several reasons. First, clear cell is present in roughly 4-6% of all endometrial cancers. As a result, there are no good statistics on just clear cell, except that it is more aggressive. Secondly, clear cell is often found in a later stage (stage 3 or 4) and that may account for the general view that it is aggressive.
Anyone who confidently gives you survival rates is making a guess: they are often just using broad statistics for endometrial cancers overall and adjusting them up subjectively. Not that reliable.
Last year (in 07), the Gynecology Oncology Group (GOG) did a huge study comparing treatment strategies that was published in one of the cancer journals. It may have some useful data given the sample size they used.
The other thing I would suggest is that you follow the research progress for other types of clear cell (Kidney clear cell being the most common), since advances there may be very useful in treatment of endometrial clear cell. Some researchers are going so far as to say that the origin of the clear cell doesn't matter, just the fact that it is clear cell cancer that should guide treatment strategies. Push your doctors hard on that front, since new drugs for kidney clear cell have been approved just in the past 12-18 months.
Good luck.