Exceedingly long time (3)
I believe the answer is:
eon
'exceedingly long time' 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 "Huge time period" , "thousand million years" , "billion years" , "Very long period (US and technical spelling)" , "Very long age" .)