Very long time period (3)
I believe the answer is:
eon
'very long time period' 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 "Immense period" , "Long period of time" , "Very long period (US and technical spelling)" , "Vast age; eternity" , "Huge time period" .)