Eons (4)
I believe the answer is:
ages
'eons' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ages that I've seen before include "A long time (colloq.)" , "Historic periods or prolonged time" , "Rock of - - (hymn)" , "Grows old like sage" , "(What seems) a very long time" .)