Top expert receives millions (4)
I believe the answer is:
acme
'top' is the definition.
(synonyms)
'expert receives millions' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'receives' means one lot of letters goes inside another (inserted letters are received).
'millions' becomes 'm' (abbreviation).
'ace' going around 'm' is 'ACME'.
(Other definitions for acme that I've seen before include "Culminating point" , "Height of achievement" , "Came awkwardly to the highest point" , "Summit - zenith" , "Peak of perfection" .)