Lengthier arc hypothetically contains vertical structure (9)
I believe the answer is:
hierarchy
'vertical structure' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'lengthier arc hypothetically contains' is the wordplay.
'contains' indicates the answer is hidden within the clue.
'HIERARCHY' can be found hidden inside 'lengthier arc hypothetically'.
Can you help me to learn more?
(Other definitions for hierarchy that I've seen before include "System with grades of authority ranking one above another" , "priesthood" , "Governing levels of organisation" , "System of status rankings" , "Leaders in church" .)