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