Vacuous apprentice working for a very long time (4)
I believe the answer is:
aeon
'very long time' is the definition.
(an aeon or eon is a long time)
'vacuous apprentice working' is the wordplay.
'vacuous' says to hollow out the word (remove centre letters).
'working' becomes 'on' (e.g. a machine is working or on).
'apprentice' with its centre removed is 'ae'.
'ae'+'on'='AEON'
'for a' is the link.
(Other definitions for aeon that I've seen before include "A very long time, an age" , "With lots of time" , "Eternity" , "An age; period" , "Vast time period" .)