Apogee, summit (4)
I believe the answer is:
apex
I believe this is a double definition.
'apogee' is the first definition.
(I've seen this in another clue)
'summit' is the second definition.
(I've seen this in another clue)
(Other definitions for apex that I've seen before include "The highest point of something" , "Uppermost tip" , "high-point" , "Top, peak" , "Absolute peak" .)