Worked in duet as one (6)
I believe the answer is:
united
'as one' is the definition.
(united as one)
'worked in duet' is the wordplay.
'worked' is an anagram indicator.
'in'+'duet'='induet'
'induet' anagrammed gives 'UNITED'.
(Other definitions for united that I've seen before include "... we stand, divided we fall" , "what may be seen after Oxford and Cambridge?" , "Joined together as in alliance" , "collective" , "Amalgamated" .)