First of mannequins wearing superb top (4)
I believe the answer is:
acme
'top' is the definition.
('acme' can be a synonym of 'top')
'first of mannequins wearing superb' is the wordplay.
'first of' suggests taking the first letters.
'wearing' is an insertion indicator.
'superb' becomes 'ace' (I've seen this in other clues).
The first letter of 'mannequins' is 'm'.
'm' put within 'ace' is 'ACME'.
(Other definitions for acme that I've seen before include "Highest stage of development" , "Point of maximum development" , "Top; peak" , "Point of perfection, apex" , "Top or highest point" .)