One dying for a cause (6)
I believe the answer is:
martyr
'one dying 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 "One dying for religious beliefs" , "One suffering for conscience" , "One suffers for the sake of principle" , "Person who dies for a cause" , "One suffering for beliefs" .)