Asian writer upset boxer (6)
I believe the answer is:
nepali
'asian' is the definition.
(from Nepal)
'writer upset boxer' is the wordplay.
'writer' becomes 'pen' (something which writes).
'upset' says the letters should be written in reverse.
'boxer' becomes 'Ali' (Muhammad Ali).
'pen' backwards is 'nep'.
'nep'+'ali'='NEPALI'
(Other definitions for nepali that I've seen before include "Mountain-dweller" , "Foreigner" , "Language of Kathmandu" , "Native of a mountainous Hindu kingdom" , "Asian national" .)