Be in a choir that covers Western and big band music (5)
I believe the answer is:
swing
'big band music' is the definition.
'swing' can be an answer for 'music' (swing music is a kind of music). I am not sure about the 'big band' bit.
'be in a choir that covers western and' is the wordplay.
'be in' means one lot of letters goes inside another.
'a choir' becomes 'sig' (I can't justify this - if you can you should believe this answer much more).
'that covers' is an insertion indicator.
'western' becomes 'w'.
'and' becomes 'n' (common abbreviation for 'and').
'sig' enclosing 'w' is 'swig'.
'swig' placed around 'n' is 'SWING'.
Can you help me to learn more?
(Other definitions for swing that I've seen before include "Sway to and fro" , "Move like a pendulum" , "Shift opinions; hang" , "Move from side to side" , "Change opinion" .)