Oscar-winning film: King Sebastian (6)
I believe the answer is:
martyr
'sebastian' is the definition.
(St Sebastian is an example)
'oscar-winning film king' is the wordplay.
'oscar-winning film' becomes 'marty'.
'king' becomes 'r' (r is abbreviation for rex).
'marty'+'r'='MARTYR'
(Other definitions for martyr that I've seen before include "Persecution victim" , "One suffers for the sake of principle" , "Sufferer for faith" , "One who dies for a cause" , "One dying for belief" .)