Victim in trade centre, unknown Republican (6)
I believe the answer is:
martyr
'victim' is the definition.
(martyr is a kind of victim)
'trade centre unknown republican' is the wordplay.
'trade centre' becomes 'mart'.
'unknown' becomes 'y' (algebra).
'republican' becomes 'r' (abbreviation).
'mart'+'y'+'r'='MARTYR'
'in' acts as a link.
(Other definitions for martyr that I've seen before include "put at stake?" , "One sacrificed to cause" , "Victim; eg St Stephen" , "One who suffers death rather than renounce their religion" , "One who dies for a cause" .)