Top primate gets cross (4)
I believe the answer is:
apex
'top' is the definition.
(thesaurus)
'primate gets cross' is the wordplay.
'primate' becomes 'ape' (I've seen this before**).
'gets' is a charade indicator (letters next to each other).
'cross' becomes 'x'.
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "Uppermost point" , "Summit - pinnacle" , "Top, highest part" , "The highest point or pointed end" , "Pointed top" .)