Not a chorister (anag.) (13)

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" .)

I've seen this clue in The Telegraph.

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