Peak or zenith (4)
I believe the answer is:
apex
I believe this clue is a double definition.
'peak' is the first definition.
(I know that peak can be written as apex)
'zenith' is the second definition.
(I've seen this in another clue)
(Other definitions for apex that I've seen before include "Pointed top" , "Apogee, summit" , "Culmination" , "Top point" , "Summit or peak" .)