Top expert receiving millions (4)
I believe the answer is:
acme
'top' is the definition.
('acme' can be a synonym of 'top')
'expert receiving millions' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'receiving' indicates putting letters inside.
'millions' becomes 'm' (abbreviation).
'ace' enclosing 'm' is 'ACME'.
(Other definitions for acme that I've seen before include "Top; peak" , "Highest level or degree attainable" , "Pinnacle" , "Peak of perfection" , "Summit - climax" .)