Performing in duet together (6)
I believe the answer is:
united
'together' is the definition.
(I've seen this before)
'performing in duet' is the wordplay.
'performing' indicates an anagram.
'in'+'duet'='induet'
'induet' is an anagram of 'UNITED'.
(Other definitions for united that I've seen before include "Leeds, Sheffield or Manchester, perhaps" , "Joined in marriage" , "joined with others" , "Whole" , "See 10" .)