thousands of years (9)
I believe the answer is:
millennia
'thousands of years' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for millennia that I've seen before include "Thousand year periods" , "2000 years minimum" , "Spans of 1,000 years" , "Thousands of years" , "Could seem an interminable time" .)