In arrangement, a Wagnerian may offer a seasonal song (4,2,1,6)
I believe the answer is:
away in a manger
'song' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for away in a manger that I've seen before include "Familiar tune" , "song?" , "Christmas carol" .)