Rings about traitor joining city composition (8)
I believe the answer is:
oratorio
'composition' is the definition.
(type of musical composition)
'rings about traitor joining city' is the wordplay.
'rings' becomes 'oo' (multiple circles).
'about' is an insertion indicator.
'traitor' becomes 'rat' (I've seen this before).
'joining city' becomes 'rio' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'oo' placed around 'rat' is 'orato'.
'orato'+'rio'='ORATORIO'
Can you help me to learn more?
(Other definitions for oratorio that I've seen before include "E.g. Messiah or Elijah" , "Comprehensive music" , "Musical composition for voices, chorus and orchestra with biblical theme" , "Music for voices and orchestra with biblical theme" , "religious music" .)