The opening note? (3)
I believe the answer is:
key
This might be a double definition.
'the opening' is the first definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'note?' is the second definition.
(thesaurus)
Can you help me to learn more?
(Other definitions for key that I've seen before include "Eg, Ctrl, Alt or Esc" , "Vital" , "Crucially important" , "Piano lever" , "Lock opener; low island" .)