Uranium in reactor upset union official (8)
I believe the answer is:
eurocrat
'union official' is the definition.
(informal term for a European Union bureaucrat)
'uranium in reactor upset' is the wordplay.
'uranium' becomes 'u' ('U' can be a synonym of 'uranium').
'in' is an insertion indicator.
'upset' indicates an anagram.
'reactor' is an anagram of 'erocrat'.
'u' put within 'erocrat' is 'EUROCRAT'.
(Other definitions for eurocrat that I've seen before include "Brussels functionary" , "Common Market official" , "Brussels mandarin" , "Brussels official - our trace (anag)" , "Brussels employee" .)