Do the same with the ten on top (4)
I believe the answer is:
apex
'top' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for apex that I've seen before include "The highest point or peak" , "Pointed top" , "white collar union" , "Summit - pinnacle" , "The highest point or pointed end" .)