Anti-slavery campaigner (12)
I believe the answer is:
abolitionist
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">anti-slavery campaigner</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for abolitionist that I've seen before include "One against, eg, slavery" , "Anti-slavery campaigner" , "One favouring end of capital punishment" , "Reformer (eg Wilberforce)" .)