40's foe (4)
I believe the answer is:
axis
'40's foe' 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 "Imaginary line around which object rotates" , "(Rotational?) alliance of powers" , "One of very elliptical orbit" , "Group of countries in special alliance" , "Central point of a rotation" .)