Leader of Israel contributed to President Bush's ultimate victory (7)
I believe the answer is:
triumph
'victory' is the definition.
('triumph' can be a synonym of 'victory')
'leader of israel contributed to president bush's ultimate' is the wordplay.
'leader of' indicates taking the first letters.
'contributed to' means one lot of letters goes inside another.
'president' becomes 'trump' (Donald Trump).
'ultimate' suggests the final letters.
The first letter of 'israel' is 'i'.
The final letter of 'bush' is 'h'.
'i' inserted into 'trump' is 'triump'.
'triump'+'h'='TRIUMPH'
(Other definitions for triumph that I've seen before include "Prove superior" , "former car manufacturer" , "Crow" , "motorbike" , "Notable achievement" .)