Top marks secured by expert (4)
I believe the answer is:
acme
'top' is the definition.
('acme' can be a synonym of 'top')
'marks secured by expert' is the wordplay.
'marks' becomes 'm' (former German currency).
'secured by' indicates putting letters inside (I've seen this in other clues).
'expert' becomes 'ace' (ace is a kind of expert).
'm' placed into 'ace' is 'ACME'.
(Other definitions for acme that I've seen before include "Summit - maximum" , "Peak of excellence" , "Culmination" , "Point of perfection, apex" , "Zenith - optimum" .)