A chorister not involved in the arrangement (13)
I believe the answer is:
orchestration
'arrangement' is the definition.
(orchestration is a kind of arrangement)
'a chorister not involved' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'a'+'chorister'+'not'='achoristernot'
'achoristernot' is an anagram of 'ORCHESTRATION'.
'in the' acts as a link.
(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" , "Composer's skill" , "Musical arrangement" , "Arrangement for instruments - short reaction (anag)" .)