A minor, for instance (3)

I believe the answer is:

key

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'a minor' is the definition.
(thesaurus)

I don't understand the remainder of the clue.

(Other definitions for key that I've seen before include "Scale system for music" , "Of crucial importance" , "Eg, Ctrl, Alt or Esc" , "A major say" , "It can open" .)

I've seen this clue in the Evening Standard.

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