Many, many years (3)
I believe the answer is:
eon
'many many years' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eon that I've seen before include "Vast age; eternity" , "Vast period" , "Very long period (US and technical spelling)" , "Age, epoch" , "Prolonged period" .)