One works for a long time (3)
I believe the answer is:
eon
'long time' is the definition.
(eon is a kind of long time)
'one works' is the wordplay.
'works' indicates an anagram (work can mean to twist or convulse).
'one' anagrammed gives 'EON'.
'for a' acts as a link.
(Other definitions for eon that I've seen before include "An immense time" , "In time, long division" , "Huge time period" , "Major division of geological time" , "Vast age (alt. sp.)" .)