A note sounded by Homer (3)
I believe the answer is:
doh
'a note' is the definition.
(alternative spelling of 'do' as in do re mi)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for doh that I've seen before include "It begins the tonic solfa" , "Silly me!" , "Stupid me!" , "First of the series at 17 Across" , "an exclamation from someone who's annoyed with himself" .)