Song rendered by idiot in yell (7)
I believe the answer is:
wassail
'song rendered' is the definition.
I can't judge whether this definition defines the answer.
'idiot in yell' is the wordplay.
'idiot' becomes 'ass' (both can mean a stupid person).
'in' is an insertion indicator.
'yell' becomes 'wail' (I can't explain this - if you can you should give a lot more credence to this answer).
'ass' placed into 'wail' is 'WASSAIL'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for wassail that I've seen before include "Lively festivities; spiced ale" , "punch" , "Drinking festivity" , "festive drink" , "Revelry" .)