Endless years (4)
I believe the answer is:
aeon
'endless years' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aeon that I've seen before include "Longest division of geological time" , "Eternity" , "Very long and indefinite period of time" , "Immeasurably long period of time" , "Vast time period" .)