Top expert banking millions (4)
I believe the answer is:
acme
'top' is the definition.
('acme' can be a synonym of 'top')
'expert banking millions' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'banking' means one lot of letters goes inside another.
'millions' becomes 'm' (abbreviation).
'ace' going around 'm' is 'ACME'.
(Other definitions for acme that I've seen before include "Point of perfection, apex" , "Summit - maximum" , "Height of achievement" , "Pinnacle" , "Top; peak" .)