Harmony is found in rejection of what's unacceptable (6)
I believe the answer is:
unison
'harmony' is the definition.
(unison is a kind of harmony)
'is found in rejection of what's unacceptable' is the wordplay.
'found in' is an insertion indicator.
'rejection of' shows that the letters should be reversed in order.
'what's unacceptable' becomes 'nonu' (I can't justify this - if you can you should believe this answer much more).
'nonu' back-to-front is 'unon'.
'is' placed within 'unon' is 'UNISON'.
Can you help me to learn more?
(Other definitions for unison that I've seen before include "As one voice" , "In agreement - like sound of choir" , "all players directed to the same pitch" , "Type of singing" , "All voices together" .)