A Wagnerian may unexpectedly sing this? (4,2,1,6)

I believe the answer is:

away in a manger

I'm a little stuck... Click here to teach me more about this clue!

'sing this?' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'a wagnerian may unexpectedly' is the wordplay.
'unexpectedly' is an anagram indicator.
'wagnerian'+'may'='wagnerianmay'
'wagnerianmay' is an anagram of 'wayinamanger'.
'a'+'wayinamanger'='AWAY IN A MANGER'

Can you help me to learn more?

(Other definitions for away in a manger that I've seen before include "song?" , "Familiar tune" , "Christmas carol" .)

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app