Union of US in disorder (6)
I believe the answer is:
fusion
'union' is the definition.
(fusion is a kind of union)
'of us in disorder' is the wordplay.
'disorder' indicates an anagram (a disordered version of the letters).
'of'+'us'+'in'='ofusin'
'ofusin' anagrammed gives 'FUSION'.
(Other definitions for fusion that I've seen before include "A welding" , "The process of joining two or more into one" , "Atomic reaction" , "Mixed musical style" , "Unification, maybe nuclear" .)