Top ten supporting primate (4)
I believe the answer is:
apex
'top' is the definition.
(apex is a top or peak)
'ten supporting primate' is the wordplay.
'ten' becomes 'X' (Roman numeral).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'primate' becomes 'ape' (large primate).
'x' put after 'ape' is 'APEX'.
(Other definitions for apex that I've seen before include "Peak, vertex" , "The tip or highest point (4)" , "Point or summit" , "The highest point of something" , "Pointed top" .)