One worked for a long time (3)
I believe the answer is:
eon
'long time' is the definition.
(eon is a kind of long time)
'one worked' is the wordplay.
'worked' indicates an anagram.
'one' with letters rearranged gives 'EON'.
'for a' is the link.
(Other definitions for eon that I've seen before include "Major division of geological time" , "billion years" , "Immense period" , "thousand million years" , "plenty of time" .)