Harmony provided by one of the strings, we hear (6)
I believe the answer is:
accord
'harmony' is the definition.
(accord is a kind of harmony)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for accord that I've seen before include "Agreement - conformity" , "binding agreement" , "Harmony, concurrence" , "Match" , "Peace and agreement" .)