Neighbouring state (8)
I believe the answer is:
'neighbouring state' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'neighbouring state' is the wordplay.
'neighbouring' becomes 'near' (similar in meaning).
'state' becomes 'ness' (I can't justify this - if you can you should give a lot more credence to this answer).
Can you help me to learn more?
(Other definitions for nearness that I've seen before include "Propinquity" , "Proximity" , "neighbour's property" , "Quality of a mean person" , "Closeness" .)