A number backing World War II alliance (4)
I believe the answer is:
axis
'world war ii alliance' is the definition.
(I've seen this in another clue)
'a number backing' is the wordplay.
'number' becomes 'six' (six is a kind of whole number).
'backing' is a reversal indicator.
'six' backwards is 'xis'.
'a'+'xis'='AXIS'
(Other definitions for axis that I've seen before include "Spotted deer" , "Fixed line in revolution" , "league" , "WW2 Nazi alliance" , "Line about which a body turns or a construction is symmetrical" .)