Joined in duet (6)
I believe the answer is:
united
'joined' is the definition.
('unite' can be a synonym of 'join')
'in duet' is the wordplay.
'in duet' anagrams to 'UNITED'.
Though, I'm unsure how this is indicated.
Can you help me to learn more?
(Other definitions for united that I've seen before include "See 9" , "Joined into a single entity" , "See 18" , "Leeds, Sheffield or Manchester, perhaps" , "Untied, being allied" .)