Not a chorister (anag.) (13)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
orchestration
(Other definitions for orchestration that I've seen before include "Arrangement of string, brass, woodwind and percussion parts" , "Scoring for strings, brass, woodwind and percussion" , "Stage-management" , "Arrangement for instruments - short reaction (anag)" , "Composer's skill" .)