Many moons (3)
'many moons' 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 "Prolonged period" , "billion years" , "Immense period" , "plenty of time" , "Vast period" .)
(Other definitions for ago that I've seen before include "(Years) before the present time" , "Back (in time)" , "Previously" , "Gone, past, since" , "Time past" .)