Top primate, cross (4)
I believe the answer is:
apex
'top' is the definition.
(thesaurus)
'primate cross' is the wordplay.
'primate' becomes 'ape' (I've seen this before**).
'cross' becomes 'x'.
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "Peak, vertex" , "Culmination" , "Top point" , "The highest point or peak" , "The highest point or pointed end" .)