Neither do we gain by changing nationality (9)
I believe the answer is:
norwegian
'changing nationality' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'neither do we gain' is the wordplay.
'neither' becomes 'nor' (eg in the phrase 'neither do I').
'do' indicates an anagram (solvers must work out or 'do' the anagram puzzle).
'we'+'gain'='wegain'
'wegain' anagrammed gives 'wegian'.
'nor'+'wegian'='NORWEGIAN'
'by' acts as a link.
Can you help me to learn more?
(Other definitions for norwegian that I've seen before include "From Oslo, perhaps" , "like Grieg" , "Northerner" , "like King Harald I" , "One may live overseas" .)