Evangelist before long beginning to rant (8)
I believe the answer is:
preacher
'evangelist' is the definition.
(I've seen this before)
'before long beginning to rant' is the wordplay.
'before' becomes 'pre'.
'long' becomes 'ache' ('ache' can be a synonym of 'long').
'beginning to' indicates taking the first letters.
The initial letter of 'rant' is 'r'.
'pre'+'ache'+'r'='PREACHER'
(Other definitions for preacher that I've seen before include "cleric" , "Sermoniser; minister" , "Minister, clergyman" , "evangelist found" , "One spreads the message" .)