Primate gets kiss at the top (4)
I believe the answer is:
Here is my best explanation:
'the top' is the definition.
(I've seen this before)
'primate gets kiss' is the subsidiary indication.
'primate' becomes 'ape' (ape is a kind of primate**).
'gets' is a charade indicator (letters next to each other).
'kiss' becomes 'x'.
'at' is the link.
(Other definitions for apex that I've seen before include "Highest point (of something)", "Top, highest part", "The highest point or peak", "Pointed top", "Absolute peak", "Apogee, summit", "The highest point or pointed end", "Peak, vertex", "Top point" and "The highest point of something".)