Anti-slavery campaigner (12)
I believe the answer is:
abolitionist
'anti-slavery campaigner' is the definition.
(I've seen this before)
This is all the clue.
(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)" .)