Special United Nations intelligence upset European Union (6)
I believe the answer is:
unique
'special' is the definition.
(thesaurus)
'united nations intelligence upset european union' is the wordplay.
'united nations' becomes 'UN' (abbreviation).
'intelligence' becomes 'IQ' (short for intelligence quotient).
'upset' is a reversal indicator.
'european union' becomes 'eu'.
'eu' reversed gives 'ue'.
'un'+'iq'+'ue'='UNIQUE'
(Other definitions for unique that I've seen before include "Radically distinctive and without equal" , "Single" , "incomparable" , "Singular, without equal" , "it's most unusual" .)