One's worked for a long time (4)
I believe the answer is:
eons
'long time' is the definition.
(I've seen this in another clue)
'one's worked' is the wordplay.
'worked' indicates an anagram.
'ones' with letters rearranged gives 'EONS'.
'for a' 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)" .)