An opener is such a man (3)

I believe the answer is:

key

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'an opener' is the definition.
(keys open things)

I don't understand how the remainder of the clue works.

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

I've seen this clue in the Evening Standard.

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