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" .)

I've seen this clue in The Telegraph.

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