Top primate put on crucifix, finally (4)
I believe the answer is:
apex
'top' is the definition.
(I've seen this before)
'primate put on crucifix finally' is the wordplay.
'primate' becomes 'ape' (ape is a kind of primate).
'put on' says to put letters next to each other.
'finally' indicates one should take the final letters.
The last letter of 'crucifix' is 'x'.
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "high-point" , "Top, highest part" , "Peak, vertex" , "Apogee, summit" , "Uppermost tip" .)