Leader of march wearing superb top (4)
I believe the answer is:
acme
'top' is the definition.
('acme' can be a synonym of 'top')
'leader of march wearing superb' is the wordplay.
'leader of' says to take the initial letters.
'wearing' is an insertion indicator.
'superb' becomes 'ace' (I've seen this in other clues).
The initial letter of 'march' is 'm'.
'm' going within 'ace' is 'ACME'.
(Other definitions for acme that I've seen before include "Summit - maximum" , "Summit of achievement" , "Summit, crowning point" , "Came awkwardly to the highest point" , "Summit - zenith" .)