The entrance to a day job (9)
I believe the answer is:
admission
'the entrance' is the definition.
(admission is a kind of entrance)
'a day job' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'job' becomes 'mission' (a mission is a job or task).
'a'+'d'+'mission'='ADMISSION'
'to' acts as a link.
(Other definitions for admission that I've seen before include "charge for entrance" , "Letting in - letting out" , "for which one may be charged" , "Entrance fee" , "Permission to enter" .)