Make harmonious (6)
I believe the answer is:
attune
'make harmonious' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for attune that I've seen before include "Harmonise or adapt, say mind to idea" , "Harmonise or synchronise" , "Bring into harmony" , "Orient" , "Adjust (to a situation)" .)