Once defeated, only some surrender (4)
I believe the answer is:
cede
'surrender' is the definition.
('cede' can be a synonym of 'surrender')
'once defeated only some' is the wordplay.
'only some' indicates a hidden word.
'CEDE' is hidden within 'once defeated'.
(Other definitions for cede that I've seen before include "Surrender or yield" , "Give away" , "Give up (eg, power)" , "Allow" , "Yield formally" .)