Father’s banked millions, that’s plain (6)
I believe the answer is:
pampas
'that's plain' is the definition.
(I've seen this before)
'father's banked millions' is the wordplay.
'father's' becomes 'papa's' (papa can mean one's father).
'banked' is an insertion indicator (inserted letters get stored or banked).
'millions' becomes 'm' (abbreviation).
'papas' placed around 'm' is 'PAMPAS'.
(Other definitions for pampas that I've seen before include "Grassy plains of Argentine" , "Vast plains of South America" , "South American plains" , "Argentina grassy plain" , "Treeless plain(s)" .)