Disciple bitter about job (7)
I believe the answer is:
apostle
'disciple' is the definition.
(the apostles were disciples of Jesus)
'bitter about job' is the wordplay.
'bitter' becomes 'ale' (type of beer or bitter).
'about' indicates putting letters inside.
'job' becomes 'post' (post is a kind of job).
'ale' enclosing 'post' is 'APOSTLE'.
(Other definitions for apostle that I've seen before include "One of the biblical twelve (7)well don" , "Vigorous campaigner" , "John, perhaps" , "Chosen disciple" , "[JAMES] , for one?" .)