Top primate in front of cross (4)
I believe the answer is:
apex
'top' is the definition.
(thesaurus)
'primate in front of cross' is the wordplay.
'primate' becomes 'ape' (I've seen this before**).
'in front of' says to put letters next to each other.
'cross' becomes 'x'.
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "Top point" , "Peak, vertex" , "The tip or highest point (4)" , "Summit or peak" , "Uppermost tip" .)