Campaigner is more vulgar about essay on the radio (8)
I believe the answer is:
crusader
'campaigner' is the definition.
(eg someone on a moral crusade)
'more vulgar about essay on the radio' is the wordplay.
'more vulgar' becomes 'cruder' (crude can mean vulgar or crass).
'about' is an insertion indicator.
'essay on the radio' becomes 'sa' (I can't explain this - if you can you should believe this answer much more).
'cruder' going around 'sa' is 'CRUSADER'.
'is' is the link.
Can you help me to learn more?
(Other definitions for crusader that I've seen before include "religious zealot" , "Champion" , "Vigorous campaigner" , "Advocate for a cause - rude scar (anag)" , "Christian warrior" .)