Many eras (3)
I believe the answer is:
eon
'many eras' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eon that I've seen before include "Vast age; eternity" , "In time, long division" , "Vast age (alt. sp.)" , "billion years" , "Immense period" .)