Behold broad smile they're stifling when principal gets lost in Wagner opera (9)
I believe the answer is:
lohengrin
'wagner opera' is the definition.
(I've seen this before)
'behold broad smile' is the wordplay.
'behold' becomes 'lo' (archaic exclamation meaning 'look!').
'broad' becomes 'hen' (I can't explain this - if you can you should give a lot more credence to this answer).
'smile' becomes 'grin' (grinning is a kind of smiling).
'lo'+'hen'+'grin'='LOHENGRIN'
'they're stifling when principal gets lost in' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for lohengrin that I've seen before include "Wagner opera" , "Wagnerian opera" , "Musical entertainment" .)
