Sorting out instrument before I emit musical sounds (10)
I believe the answer is:
organising
'sorting out' is the definition.
'instrument before i emit musical sounds' is the wordplay.
'instrument' becomes 'organ' (organ is a kind of instrument).
'before' means one lot of letters go next to another.
'emit musical sounds' becomes 'sing' (to sing is to produce musical sounds).
'organ'+'i'+'sing'='ORGANISING'
(Other definitions for organising that I've seen before include "arrangement" , "Systematically arranging" .)