A small number backing WWII alliance (4)
I believe the answer is:
axis
'wwii alliance' is the definition.
(I've seen this before)
'a small number backing' is the wordplay.
'small number' becomes 'six'.
'backing' is a reversal indicator.
'six' reversed gives 'xis'.
'a'+'xis'='AXIS'
(Other definitions for axis that I've seen before include "Feasible" , "from Germany and Italy, originally" , "Turning point" , "WW2 Nazi alliance" , "Center around which something rotates" .)