The solution, note (3)

I believe the answer is:

key

This clue is a double definition.

'the solution' is the first definition.
(thesaurus)

'note' is the second definition.
(thesaurus)

(Other definitions for key that I've seen before include "locker" , "It can open" , "(3)" , "explanation" , "Crucial arch-stone" .)

I've seen this clue in the Evening Standard.

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