Allies' rival (4)
I believe the answer is:
axis
'allies' rival' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for axis that I've seen before include "Line through the middle" , "Imaginary centre of rotation" , "On which a body spins" , "WW2 Nazi alliance" , "all revolves around this" .)