One worked for a long time (3)
I believe the answer is:
eon
'long time' is the definition.
(I know that eon is a type of long time)
'one worked' is the wordplay.
'worked' indicates anagramming the letters.
'one' with letters rearranged gives 'EON'.
'for a' acts as a 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" .)