Most of this is about a nationality (4)
I believe the answer is:
thai
'nationality' is the definition.
(I've seen this before)
'most of this is about a' is the wordplay.
'most of' means to remove the last letter.
'is about' indicates putting letters inside.
'this' with its final letter taken off is 'thi'.
'thi' placed around 'a' is 'THAI'.
(Other definitions for thai that I've seen before include "East Asian" , "Sort of massage" , "Eg, native of Bangkok" , "Someone from Bangkok, perhaps" , "SE Asian" .)