Primate to cross mountaintop (4)
I believe the answer is:
apex
'mountaintop' is the definition.
(I've seen this in another clue)
'primate to cross' is the wordplay.
'primate' becomes 'ape' (large primate).
'to cross' becomes 'x' (X is cross-shaped).
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "Uppermost tip" , "Top, peak" , "high-point" , "The highest point or peak" , "Culmination" .)