Crossword Genius

Top primate sitting on value that's unknown (4)

Ross

I believe the answer is:

apex

'top' is the definition.
(apex is a top or peak)

'primate sitting on value that's unknown' is the wordplay.
'primate' becomes 'ape' (ape is a kind of primate).
'sitting on' says to put letters next to each other.
'value that's unknown' becomes 'x' (algebra).
'ape'+'x'='APEX'

(Other definitions for apex that I've seen before include "The tip or highest point (4)" , "reasonable fare" , "Cheap fare" , "The highest point or peak" , "Peak, vertex" .)

I've seen this clue in The Guardian.
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