Crossword Genius

The opening note? (3)

Ross

I believe the answer is:

key

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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" .)

I've seen this clue in the Evening Standard.
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