One country is in to return assistance (5)
I believe the answer is:
india
'one country' is the definition.
(India is an example)
'in to return assistance' is the wordplay.
'to return' shows that the letters should be reversed in order.
'assistance' becomes 'aid' (synonyms).
'aid' written backwards gives 'dia'.
'in'+'dia'='INDIA'
'is' acts as a link.
(Other definitions for india that I've seen before include "Eastern country" , "Commonwealth state" , "Kiss; quarter of a bushel" , "Country; I (radio alphabet)" , "(Part of) subcontinent" .)