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