International organisation only? That's not fair! (6)
I believe the answer is:
unjust
'not fair' is the definition.
(I know this)
'international organisation only?' is the wordplay.
'international organisation' becomes 'UN' (United Nations).
'only?' becomes 'just' (synonyms).
'un'+'just'='UNJUST'
'that's' is the link.
(Other definitions for unjust that I've seen before include "Not fair or equitable" , "Inequitable" , "Unfair" , "Biased" , "Partisan - not fair" .)