Eternity, vast age (3)
I believe the answer is:
eon
I believe this clue is a double definition.
'eternity' is the first definition.
(I've seen this before)
'vast age' is the second definition.
(I've seen this in another clue)
(Other definitions for eon that I've seen before include "plenty of time" , "Very long age" , "Vast age; eternity" , "Immense period" , "Major division of geological time" .)