Only 50 per cent in battle (6)
I believe the answer is:
midway
I believe this is a double definition.
'only 50 per cent' is the first definition.
(only halfway through)
'battle' is the second definition.
(I know that Midway is a naval battle)
'in' acts as a link.
(Other definitions for midway that I've seen before include "1942 naval battle in the Pacific - towards the centre" , "Towards the middle" , "Half the distance" , "Equally distant from the extremes" , "With as far to go as already travelled" .)