Ne plus ultra (4)
'ne plus ultra' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for acme that I've seen before include "High point of excellence" , "highest position" , "crisis in history" , "crown" , "Highest stage of development" .)
(Other definitions for apex that I've seen before include "Point or summit" , "The tip or highest point (4)" , "Top, peak" , "white collar union" , "Top point" .)