Close friend certain East will go for ace (9)

I believe the answer is:

confidant

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

I cannot understand how the rest of the clue works.

(Other definitions for confidant that I've seen before include "Trusted friend" , "intimate" , "Sharer of secrets" , "Close friend entrusted with secrets etc" .)

I've seen this clue in The Telegraph.

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