Metal instrument harmonised (6)
I believe the answer is:
agreed
'harmonised' is the definition.
('agree' can be a synonym of 'harmonize')
'metal instrument' is the wordplay.
'metal' becomes 'ag' (silver).
'instrument' becomes 'reed' (reed is a kind of instrument).
'ag'+'reed'='AGREED'
(Other definitions for agreed that I've seen before include "Done!" , "Accorded" , "In accord" , "granted" , "Saw eye to eye" .)