A good musical instrument chimed (6)
I believe the answer is:
agreed
'chimed' is the definition.
'a good musical instrument' is the wordplay.
'a good' becomes 'ag' (I've seen this in other clues).
'musical instrument' becomes 'reed' (reed is a kind of musical instrument).
'ag'+'reed'='AGREED'
(Other definitions for agreed that I've seen before include "matched" , "ratified" , "acceptable" , "Let's do it!" , "Shared an opinion" .)