Note put in to request supply of another instrument (8)
I believe the answer is:
recorder
'supply of another instrument' is the definition.
'recorder' can be an answer for 'instrument' (recorder is a kind of instrument). I am not certain of the 'supply of another' bit.
'note put in to request' is the wordplay.
'note put in' becomes 'rec' (I can't explain this - if you can you should give a lot more credence to this answer).
'to request' becomes 'order' (an order is a request from someone in authority).
'rec'+'order'='RECORDER'
Can you help me to learn more?
(Other definitions for recorder that I've seen before include "The one who takes notes and a tubular wind instrument" , "Renaissance flute" , "Annalist" , "found on the bench" , "Judge - musical instrument" .)