Champion bitter about job? (7)
I believe the answer is:
apostle
'champion' is the definition.
(I know that apostle is a type of believer)
'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' going around 'post' is 'APOSTLE'.
(Other definitions for apostle that I've seen before include "Luke or John, say" , "Evangelist, proponent" , "Vigorous campaigner" , "Originally, one sent to preach the gospel" , "eg John, Paul" .)