The highest point (6)
(Other definitions for acme that I've seen before include "Summit of achievement" , "crown" , "Highest stage of development" , "Culminating point" , "Summit - zenith" .)
'the highest point' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for apex that I've seen before include "Topmost point" , "white collar union" , "Summit or peak" , "Acme" , "Peak, vertex" .)
(Other definitions for tiptop that I've seen before include "The very highest point" , "Quality" , "First-class" , "Best of its kind" , "The height of excellence" .)