Top mimic ending on box (4)
I believe the answer is:
apex
'top' is the definition.
(apex is a top or peak)
'mimic ending on box' is the wordplay.
'mimic' becomes 'ape' (I've seen this in another clue).
'ending on' says to take the final letters.
The last letter of 'box' is 'x'.
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "Acme" , "The highest point or pointed end" , "Top point" , "Cheap fare" , "The tip or highest point (4)" .)