Point of transition (4)
I believe the answer is:
cusp
'point of transition' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cusp that I've seen before include "Pointed end (e.g. on tooth)" , "One point of a crescent moon" , "Pointed meeting of curves" , "Point (e.g. of tooth)" , "Point formed by two intersecting curves - point of transition" .)