Important access tool (3)

I believe the answer is:

key

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This clue could be a double definition.

'important' is the first definition.
(similar in meaning)

'access tool' is the second definition.
'key' can be an answer for 'access' (thesaurus). I am not certain of the 'tool' bit.

Can you help me to learn more?

(Other definitions for key that I've seen before include "Of crucial importance" , "Eg, Ctrl, Alt or Esc" , "Coral reef off southern coast of Florida" , "Clue" , "It opens the door" .)

I've seen this clue in The Times.

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