A good saxophone, for one, was in harmony (6)
I believe the answer is:
agreed
'was in harmony' is the definition.
'a good saxophone for one' is the wordplay.
'a good' becomes 'ag' (I've seen this in other clues).
'saxophone for one' becomes 'reed' (saxophone is a kind of reed).
'ag'+'reed'='AGREED'
(Other definitions for agreed that I've seen before include "Held the same opinion" , "acceptable" , "In accord" , "In harmony" , "Jointly decided" .)