Rejected Britain: serve one foreigner (7)
I believe the answer is:
kuwaiti
'foreigner' is the definition.
'rejected britain serve one' is the wordplay.
'rejected' says the letters should be written in reverse.
'britain' becomes 'uk' ('UK' can be a synonym of 'britain').
'serve' becomes 'wait'.
'one' becomes 'i' (Roman numeral).
'uk' reversed gives 'ku'.
'ku'+'wait'+'i'='KUWAITI'
(Other definitions for kuwaiti that I've seen before include "From an Eastern Arabian country" , "Gulf state citizen" , "Arab" , "Native of a Gulf state" .)