Joan of Arc, for one (6)
I believe the answer is:
martyr
'joan of arc for one' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for martyr that I've seen before include "One suffering for beliefs" , "One who suffers death rather than renounce their religion" , "One who suffers for beliefs" , "One killed for cause" , "One suffering or dying for a cause" .)