Bitter about letters, supporter of cause (7)
I believe the answer is:
apostle
'supporter of cause' is the definition.
'bitter about letters' is the wordplay.
'bitter' becomes 'ale' ('ale' can be a synonym of 'bitter').
'about' indicates putting letters inside.
'letters' becomes 'post' (I've seen this before).
'ale' placed around 'post' is 'APOSTLE'.
(Other definitions for apostle that I've seen before include "Thaddeus, perhaps" , "Divine messenger" , "Religious messenger" , "One of Christ's 12 original disciples" , "[JAMES] , for one?" .)