A big shot at Old Trafford backs alliance (4)
I believe the answer is:
axis
'alliance' is the definition.
(alliance between two countries)
'a big shot at old trafford backs' is the wordplay.
'big shot at old trafford' becomes 'six' (cricket).
'backs' says the letters should be written backwards.
'six' written backwards gives 'xis'.
'a'+'xis'='AXIS'
(Other definitions for axis that I've seen before include "WW2 Nazi alliance" , "Fixed line in revolution" , "Imaginary line around which a body rotates" , "Line around which rotation occurs" , "Central point of a rotation" .)