Rat-arsed group breaks into chorus (8)
I believe the answer is:
steaming
'rat-arsed group breaks into chorus' is the definition.
I can't tell whether this definition defines the answer.
'group breaks into chorus' is the wordplay.
'group' becomes 'team' (team is a kind of group**).
'breaks into' indicates putting letters inside.
'chorus' becomes 'sing' (I've seen this before).
'team' placed within 'sing' is 'STEAMING'.
Can you help me to learn more?
(Other definitions for steaming that I've seen before include "Very hot; angry" , "Emitting moisture in the form of vapour" , "Piping hot" , "Cooking with vapour" , "Very angry; cooking" .)