Alliance of powers (4)
I believe the answer is:
axis
'alliance of powers' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for axis that I've seen before include "Imaginary centre of rotation" , "Imaginary central line" , "Imaginary line about which a body rotates and a Dublin theatre" , "One of very elliptical orbit" , "Line of symmetry" .)