Arranging musical instrument, one with whistle (10)
I believe the answer is:
organising
'arranging' is the definition.
(organising is a kind of arranging)
'musical instrument one with whistle' is the wordplay.
'musical instrument' becomes 'organ' (organ is a kind of musical instrument**).
'one' becomes 'i' (Roman numeral).
'with' says to put letters next to each other.
'whistle' becomes 'sing' ('sing' can be a synonym of 'whistle').
'organ'+'i'+'sing'='ORGANISING'
(Other definitions for organising that I've seen before include "Systematically arranging" , "arrangement" .)