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' indicates putting letters inside.
'essay on the radio' becomes 'sa' (I am not sure about this - if you are sure 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" .)