Opposite of war (5)
I believe the answer is:
peace
'opposite of war' is the definition.
('peace' is the opposite to 'war')
This is all the clue.
(Other definitions for peace that I've seen before include "Quiet - lack of warfare" , "Calm, serenity" , "Absence of conflict" , "'Calm, harmony (5)'" , "''And I shall have some ... there, for ... comes dropping slow'' (WB Yeats)" .)