One who dies for a cause (6)
I believe the answer is:
martyr
'one who dies for a cause' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for martyr that I've seen before include "Sufferer for faith" , "Victim of suffering" , "put at stake?" , "One who dies for a cause" , "One who gives his life for a cause" .)