Top marks received by expert (4)
I believe the answer is:
acme
'top' is the definition.
(synonyms)
'marks received by expert' is the wordplay.
'marks' becomes 'M' (former German currency).
'received by' is an insertion indicator.
'expert' becomes 'ace' (ace can informally mean very good).
'm' going into 'ace' is 'ACME'.
(Other definitions for acme that I've seen before include "Highest level or degree attainable" , "Summit - zenith" , "Point of perfection, apex" , "Height of perfection" , "Point of maximum development" .)