One worked for a long time (3)
I believe the answer is:
'a long time' is the definition.
(I've seen this before)
'one worked' is the wordplay.
'worked' indicates anagramming the letters.
'one' anagrammed gives 'EON'.
'for' is the link.
(Other definitions for eon that I've seen before include "Very long period (US and technical spelling)", "plenty of time", "Very long age", "An immense time", "Era".)