Start of symphony with instrument producing note (5)
I believe the answer is:
sharp
'note' is the definition.
(thesaurus)
'start of symphony with instrument' is the wordplay.
'start of symphony' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'with' says to put letters next to each other.
'instrument' becomes 'harp' (harp is a kind of instrument).
's'+'harp'='SHARP'
'producing' is the link.
Can you help me to learn more?
(Other definitions for sharp that I've seen before include "Keen - exactly" , "British musician" , "Clearly defined; acidic" , "Quick; pointed" , "Good cutting edge or acute road turn" .)