One's worked for a long time (4)
I believe the answer is:
'a long time' is the definition.
(I've seen this in another clue)
'one's worked' is the wordplay.
'worked' indicates anagramming the letters.
'ones' is an anagram of 'EONS'.
'for' acts as a link.
(Other definitions for eons that I've seen before include "Vast ages" , "Divisions of geological time" , "Divisions of archaeological time" , "Endless periods" , "Very long periods (US)" .)