A crowning achievement (10)
I believe the answer is:
coronation
This could be a double definition.
'a crowning' is the first definition.
(I've seen this before)
'achievement' is the second definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
Can you help me to learn more?
(Other definitions for coronation that I've seen before include "State occasion" , "Ceremony of installing a monarch" , "Enthronement" , "Royal ceremony of 1953" , "street?" .)