Relative extremely generous about investing foreign cash (8)
I believe the answer is:
grandson
'relative' is the definition.
(grandson is a type of relative)
'extremely generous about investing foreign cash' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'about' becomes 'on' (as in a book 'on' a certain topic).
'investing' means one lot of letters goes inside another (inserted letters are taken in or invested).
'foreign cash' becomes 'rand' (currency of South Africa).
'generous' with its middle taken out is 'gs'.
'gs'+'on'='gson'
'gson' enclosing 'rand' is 'GRANDSON'.
(Other definitions for grandson that I've seen before include "Junior relative" , "Male offspring, third generation (8)" , "Male descendent" , "Young male descendant" , "Young male relative" .)