Highest point, apex (4)
I believe the answer is:
acme
I believe this clue is a double definition.
'highest point' is the first definition.
(I've seen this in another clue)
'apex' is the second definition.
(synonyms)
(Other definitions for acme that I've seen before include "Summit - zenith" , "Highest level or degree attainable" , "Highest point of excellence or achievement" , "high peak" , "Peak of perfection" .)