Instrument missing middle note? Choose again (7)
I believe the answer is:
reorder
'choose again' is the definition.
(eg choosing a new order at a restaurant)
'instrument missing middle note?' is the wordplay.
'instrument' becomes 'recorder' (recorder is a kind of instrument).
'missing' is a deletion indicator.
'middle note?' becomes 'C' (as in middle C on a piano).
'recorder' with 'c' taken out is 'REORDER'.
(Other definitions for reorder that I've seen before include "Make a further request" , "Arrange afresh" , "Rearrange" , "make new disposition" , "requisition another!" .)