A source of wealth, bigwig backed Antipodean soldier largely (7)
I believe the answer is:
bonanza
'a source of wealth' is the definition.
(I've seen this before)
'bigwig backed antipodean soldier largely' is the wordplay.
'bigwig' becomes 'nob' (both can mean a wealthy person).
'backed' says the letters should be written backwards.
'antipodean soldier' becomes 'anzac' (I've seen this before).
'largely' means to remove the last letter.
'nob' backwards is 'bon'.
'anzac' with its final letter removed is 'anza'.
'bon'+'anza'='BONANZA'
(Other definitions for bonanza that I've seen before include "Sudden influx of wealth" , "Event creating a sudden increase in wealth" , "Rich vein of gold or a sudden windfall" , "old cowboy series on TV" , "Unexpected wealth or luck" .)