I have never heard the ripe seed head called a "clock" until today.  From what I can tell, it comes from a child's game where the number of blows to remove all the seeds being the time.  This strikes me as an "un-encyclopedic" word choice, rather like calling an apple stem a "husband-picker" (hold onto the stem and twist once for each letter of the alphabet, your husband's name will start with the letter you said that the stem came off).  I'm guessing it's almost never past 3 o'clock and all Williams are doomed to remain single.