International organisation, one with something banal that can't be believed in! (7)
I believe the answer is:
unicorn
'that can't be believed in' is the definition.
(imaginary creature)
'international organisation one with something banal' is the wordplay.
'international organisation' becomes 'un'.
'one' becomes 'I' (Roman numeral).
'with' is a charade indicator (letters next to each other).
'something banal' becomes 'corn' (corn can mean something banal or corny).
'un'+'i'+'corn'='UNICORN'
(Other definitions for unicorn that I've seen before include "Representative of Scotland" , "Fabulous animal like horse with horn" , "Singularly horny creature of fantasy" , "Animal of fable" , "Mythical animal like horse with horn" .)