Bonanza (8)
'bonanza' is the definition.
(I know that bonanza can be written as windfall)
This is all the clue.
(Other definitions for windfall that I've seen before include "Unexpected gain" , "Fruit that has fallen from the tree or sudden good fortune" , "An unexpected piece of luck - maybe an apple" , "Godsend" , "Bruised apple" .)
(Other definitions for jackpot that I've seen before include "Large cash prize" , "more than just change for the better" , "The big pay-out - hit it" , "lot of money" , "Accumulated prize" .)