Chorister apt to go wrong in a musical environment (9,3)
I believe the answer is:
orchestra pit
'a musical environment' is the definition.
(area for musicians)
'chorister apt to go wrong' is the wordplay.
'to go wrong' indicates an anagram.
'chorister'+'apt'='choristerapt'
'choristerapt' with letters rearranged gives 'ORCHESTRA PIT'.
'in' acts as a link.
(Other definitions for orchestra pit that I've seen before include "wind down in one" , "Sunken area for theatre musicians" , "where band plays" , "space for musicians" , "Place for theatre musicians" .)