When you power a Dispenser/Dropper with a Comparator-Clock it will fire once. If the clock is at least 3 Blocks away it works. I've built a test environment (seen in the screenshots) to test at what Redstone power level this happens. It seems to work, when the comparator is 3 Blocks away from the Dispenser/Dropper. Tested this on 1.6.4 and 13w41b