Get another instrument — there's a note missing (7)
I believe the answer is:
reorder
'get another' is the definition.
(reorder can mean to get another of an item)
'instrument there's a note missing' is the wordplay.
I cannot really see how this works, but
'instrument' could be 'order' (order is a kind of instrument) and 'order' is located in the answer.
'note' could be 're' (as in do-re-mi) and 're' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
(Other definitions for reorder that I've seen before include "Again apply for" , "Send off again for" , "Make a further request" , "requisition another!" , "Rearrange" .)