Tool is for an alliance of powers (4)
I believe the answer is:
axis
'an alliance of powers' is the definition.
(I've seen this before)
'tool is' is the wordplay.
'tool' becomes 'ax' (ax is a kind of tool).
'ax'+'is'='AXIS'
'for' acts as a link.
(Other definitions for axis that I've seen before include "x or y line on a graph" , "Center around which something rotates" , "Body's line of rotation" , "Bloc" , "Feasible" .)