Campaigner (8)
I believe the answer is:
crusader
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">campaigner</span>' is the definition.<br/ ><span class="explanation-format__explanation">(eg someone on a moral crusade)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for crusader that I've seen before include "Christian warrior" , "An old soldier" , "Earnest campaigner" , "Campaigner -- rude scar (anag)" , "RA cursed (anag) - one fighting for a cause" .)