Period, age (3)
I believe the answer is:
era
This clue is a double definition.
'period' is the first definition.
(I know that era is a type of period)
'age' is the second definition.
(both can mean a long time)
(Other definitions for era that I've seen before include "Epoch, period" , "Important historical period" , "Distinct period in history" , "A particular period of time" , "8" .)