Union of us in dispute (6)
I believe the answer is:
fusion
'union' is the definition.
(fusion is a kind of union)
'of us in dispute' is the wordplay.
'dispute' indicates an anagram (I've seen 'disputed' mean this).
'of'+'us'+'in'='ofusin'
'ofusin' anagrammed gives 'FUSION'.
(Other definitions for fusion that I've seen before include "The blending of different things into a whole" , "Combination into one" , "Atomic reaction" , "This kind of music" , "A welding" .)