Revealed orchestra's first duet played ... (5)
I believe the answer is:
'revealed' is the definition.
(outing is a kind of revealing)
'orchestra's first duet played' is the wordplay.
'orchestra's first' becomes 'o' (1st letter of 'orchestra').
'played' indicates an anagram.
'duet' with letters rearranged gives 'uted'.
(Other definitions for outed that I've seen before include "Exposed in public" , "Exposed against one's will" , "Revealed (someone) as gay" , "Revealed the homosexuality of (someone)" .)