Top primate seen with cross (4)
I believe the answer is:
apex
'top' is the definition.
(apex is a top or peak)
'primate seen with cross' is the wordplay.
'primate' becomes 'ape' (large primate).
'seen with' says to put letters next to each other.
'cross' becomes 'x' (an X is a cross).
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "The tip or highest point (4)" , "Absolute peak" , "Top, peak" , "reasonable fare" , "crown" .)