Alliance backing boundary area (4)
I believe the answer is:
axis
'alliance' is the definition.
(alliance between two countries)
'backing boundary area' is the wordplay.
'backing' says the letters should be written backwards.
'boundary' becomes 'six' (type of score in cricket).
'area' becomes 'a' (maths abbreviation).
'six'+'a'='sixa'
'sixa' written backwards gives 'AXIS'.
(Other definitions for axis that I've seen before include "Imaginary dividing line" , "Center around which something rotates" , "Pivot, political alliance" , "Imaginary line around which object rotates" , "line to be turned round" .)