Big windfall (7)
I believe the answer is:
bonanza
'big windfall' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for bonanza that I've seen before include "A sudden happening that brings good fortune" , "Good source of profits" , "Rich vein of gold or a sudden windfall" , "Sudden and unexpected source of wealth" , "Huge stroke of luck" .)