Sudden windfall (7)
I believe the answer is:
bonanza
'sudden windfall' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for bonanza that I've seen before include "Mine of wealth" , "Source of great and sudden wealth" , "Source of wealth or good fortune" , "Huge stroke of luck" , "Sudden and unexpected source of wealth" .)