A long, long time
'a long long time' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eons that I've seen before include "Times" , "Divisions of geological time" , "Vast ages" , "Endless periods" , "Long periods of time - noes have it" .)
I believe this is a double definition.
'a long' is the first definition.
(thesaurus)
'long time' is the second definition.
(I've seen this before)
(Other definitions for ages that I've seen before include "Long periods" , "Rock of - - (hymn)" , "Awfully long time" , "Epochs" , "A long time (colloq.)" .)
(Other definitions for decades that I've seen before include "Ten-year spells" , "10-year periods" , "Periods of 10 years" , "Ten year periods" .)