A number backing an alliance (4)
I believe the answer is:
axis
'an alliance' is the definition.
(alliance between two countries)
'a number backing' is the wordplay.
'number' becomes 'six' (six is a kind of number).
'backing' says the letters should be written backwards.
'six' backwards is 'xis'.
'a'+'xis'='AXIS'
(Other definitions for axis that I've seen before include "Alliance of powers eg Germany, Italy and Japan in WWII" , "On which a body spins" , "Spindle - German/Italian alliance - deer" , "Line from N. to S. Pole" , "all revolves around this" .)