Why would you want to do that? It is like driving a Porsche, but only using first gear.
Two [potential] reasons -- to run a cooler/quieter box when the ambient temperature climbs (NY is more and more like Cairo every year), and since OC'ing seems to give back relatively small gains, particularly for timeline editing. A 10-15% gain for a task (e.g., exporting), which I don't often perform may not be worth the increased load.
Actually, you will not gain anywhere near that much in the way of cooler and quieter operation when you underclock a CPU. In fact, you will lose far more in total performance than you will gain in coolness and quietness. Some time ago I performed a test with my main system at both 3.9GHz ("Turbo'd"), 3.4GHz and 1.6GHz ("fail-safe"). As far as the CPU temperatures, there is barely any difference at all whatsoever in the maximum or minimum CPU temperatures (no more than about 2 to 3 degrees Celsius) between the three clock speed settings. What's more, the CPU cooling fan runs equally as "loud" at 1.6GHz as it does at 3.9GHz. (This is all because modern CPUs have a certain required minimum voltage just to even function at all; lowering that voltage below that minimum resulted in the system failing to even POST at even the minimum fail-safe clock speed.)
Thus, if you think that your CPU is going to overheat (or at least run significantly hotter than normal) at the default Turbo settings, then you should look into replacing your current CPU cooler with a better performing one.