Elderly relative in Dickens getting a windfall (7)
I believe the answer is:
bonanza
'windfall' is the definition.
(situation leading to an increase in wealth)
'elderly relative in dickens getting a' is the wordplay.
'elderly relative' becomes 'nan' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'dickens' becomes 'boz' (pseudonym of Charles Dickens in 'Sketches by Boz').
'getting' means one lot of letters go next to another.
'nan' placed inside 'boz' is 'bonanz'.
'bonanz'+'a'='BONANZA'
(Other definitions for bonanza that I've seen before include "Source of good luck or wealth" , "Sudden and unexpected source of wealth" , "instant prosperity" , "Source of great and sudden wealth" , "A sudden happening that brings good fortune" .)