Top primate with cross (4)
I believe the answer is:
apex
'top' is the definition.
(apex is a top or peak)
'primate with cross' is the wordplay.
'primate' becomes 'ape' (large primate).
'with' is a charade indicator (letters next to each other).
'cross' becomes 'x' (X is cross-shaped).
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "white collar union" , "climax" , "Culmination" , "Uppermost tip" , "Cheap fare" .)