Son admitted to proper nationality (7)
I believe the answer is:
descent
'nationality' is the definition.
'son admitted to proper' is the wordplay.
'son' becomes 's'.
'admitted to' means one lot of letters goes inside another.
'proper' becomes 'decent' ('decent' is associated in meaning with 'proper').
's' put within 'decent' is 'DESCENT'.
(Other definitions for descent that I've seen before include "Comedown (of ancestry?)" , "Fall" , "Downward journey" , "Downward path" , "Ancestry - downward move" .)