Duet in discord or in harmony (6)
I believe the answer is:
united
'in harmony' is the definition.
(I've seen this in another clue)
'duet in discord' is the wordplay.
'discord' indicates anagramming the letters.
'duet'+'in'='duetin'
'duetin' is an anagram of 'UNITED'.
'or' is the link.
(Other definitions for united that I've seen before include "Joined into a single entity" , "Manchester _____, current Premiership champions with..." , "See 10" , "joined with others" , "Joined together, allied" .)