Not bound to change in duet (6)
I believe the answer is:
untied
'not bound' is the definition.
('untie' is the opposite to 'bind')
'change in duet' is the wordplay.
'change' indicates anagramming the letters.
'in'+'duet'='induet'
'induet' with letters rearranged gives 'UNTIED'.
'to' acts as a link.
(Other definitions for untied that I've seen before include "No longer bound" , "Undid the knot" , "Picked loose" , "Unfastened, knot say" , "Lacking any fetters" .)