Sang dully in key, including bum note (7)
I believe the answer is:
intoned
'sang dully' is the definition.
'intoned' can be an answer for 'sang' (intoning is a kind of singing). I'm unsure of the 'dully' bit.
'in key including bum note' is the wordplay.
'key' becomes 'd' (musical key).
'including' means one lot of letters goes inside another.
'bum' indicates anagramming the letters (bum can mean faulty or wrong).
'in'+'d'='ind'
'note' with letters rearranged gives 'tone'.
'ind' going around 'tone' is 'INTONED'.
Can you help me to learn more?
(Other definitions for intoned that I've seen before include "Recited almost monotonically" , "sounded like the vicar?" , "Sang in church" , "it's said, in a monotonous way" , "Started a psalm, perhaps" .)