European steals in, after forging key (9)

I believe the answer is:

essential

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'key' is the definition.
(I've seen this before)

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

(Other definitions for essential that I've seen before include "something necessary" , "Vitally important" , "So is certain oil-production" , "Basic or necessary" , "Indispensable, necessary" .)

I've seen this clue in The Times.

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