A clock signal is defined by clock period or clock frequency. The clock period is defined as time interval after which clock repeats it self or we can define it as time gap between two consecutive falling edges or two consecutive rising edges and clock frequency is defined as number of clock pulses in a second.
Clock freq = 1/ clock period
Duty Cycle: of a periodic wave is defined as percentage of the clock period we have a HIGH pulse.
i.e. Duty cycle = (time for which pulse is 1)*100 / Clock period
If width of HIGH pulse = width of LOW pulse = t as shown above.
Then Duty cycle = t/ 2t *100 = 50%
If we are given duty cycle = 33%, then it means
100/3 = (time for which pulse is 1)*100 / Clock period
Hence width of HIGH pulse = (1/3) * clock period as shown below: