Transition point (4)
I believe the answer is:
cusp
'transition point' 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 "Turning point on which important developments depend" , "Prominence on a tooth" , "Point (e.g. of tooth)" , "Horn of moon" , "One point of a crescent moon" .)