Relative wants us to invest in currency (6)
I believe the answer is:
cousin
'relative' is the definition.
(I've seen this before)
'us to invest in currency' is the wordplay.
'to invest in' means one lot of letters goes inside another.
'currency' becomes 'coin' (coin is a kind of currency).
'us' inserted inside 'coin' is 'COUSIN'.
'wants' is the link.
(Other definitions for cousin that I've seen before include "Kinsman, generally (archaic)" , "US icon is an odd relative" , "Child of one's uncle or aunt" , "Aunt's/uncle's child" , "person once removed?" .)