Rich person giving away capital? That’s not on (3)
I believe the answer is:
off
'that's not on' is the definition.
('off' is the opposite to 'on')
'rich person giving away capital?' is the wordplay.
'rich person' becomes 'toff' (toff is a kind of rich person).
'giving away capital?' means to remove the first letter (without the capital letter).
'toff' with its initial letter removed is 'OFF'.
(Other definitions for off that I've seen before include "(Of food) no longer fresh" , "Unsatisfactory" , "Of food, begining to decay" , "Not on much 16 down" , "No longer available" .)