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