The French woman ultimately wielding supreme power (8)
I believe the answer is:
leverage
'power' is the definition.
(I've seen this before)
'the french woman ultimately wielding supreme' is the wordplay.
'the french' becomes 'le' ('the' in French).
'woman' becomes 'vera' (a woman's name).
'ultimately' indicates one should take the final letters.
The final letters of 'wielding supreme' are 'ge'.
'le'+'vera'+'ge'='LEVERAGE'
(Other definitions for leverage that I've seen before include "financial advantage" , "Power -- advantage" , "Capacity to exert pressure" , "Power to act effectively" , "What crowbar provides" .)