Dispute of us in a union (6)
I believe the answer is:
fusion
'a union' is the definition.
(I know that fusion is a type of union)
'dispute of us in' is the wordplay.
'dispute' indicates an anagram (I've seen 'disputed' mean this).
'of'+'us'+'in'='ofusin'
'ofusin' is an anagram of 'FUSION'.
(Other definitions for fusion that I've seen before include "This kind of music" , "Blend" , "Act of joining as one" , "Combination into one" , "Act of melting or joining together" .)