Not against characters getting a rise in War Office (3)
I believe the answer is:
for
'not against' is the definition.
(I've seen this in another clue)
'characters getting a rise in war office' is the wordplay.
'getting a rise' says the letters should be written in reverse (in a down clue, letters go up).
'characters' indicates the answer is hidden within the clue (some letters or characters from the clue).
'rof' is hidden within 'in war office'.
'rof' backwards is 'FOR'.
(Other definitions for for that I've seen before include "In support of" , "Favouring" , "On the side of" , "See 24" , "On behalf of" .)