In truth, war tends to frustrate (6)
I believe the answer is:
thwart
'to frustrate' is the definition.
('thwart' can be a synonym of 'frustrate')
'in truth war tends' is the wordplay.
'in' says the answer is hidden in the clue.
'THWART' can be found hidden inside 'truth war tends'.
(Other definitions for thwart that I've seen before include "Frustrate, prevent from doing" , "Prevent from accomplishing something" , "seat for oarsman" , "Defeat" , "Frustrate and obstruct" .)