High point’s end, not start (3)
I believe the answer is:
top
'high point's' is the definition.
(I've seen this in another clue)
'end not start' is the wordplay.
'end' becomes 'stop' (stopping is a kind of ending).
'not start' means to remove the first letter.
'stop' with its initial letter removed is 'TOP'.
(Other definitions for top that I've seen before include "Zenith" , "Highest point or position" , "Uppermost part" , "Pinnacle" , "The upper part" .)