Disciple bitter about situation (7)
I believe the answer is:
apostle
'disciple' is the definition.
(the apostles were disciples of Jesus)
'bitter about situation' is the wordplay.
'bitter' becomes 'ale' (type of beer or bitter).
'about' means one lot of letters goes inside another.
'situation' becomes 'post' (both can mean a job or position).
'ale' going around 'post' is 'APOSTLE'.
(Other definitions for apostle that I've seen before include "A preacher of gospels" , "Chosen disciple" , "Maybe Peter" , "Vigorous supporter" , "Early Christian teacher" .)