Close friend certain East will go for ace (9)
I believe the answer is:
confidant
'close friend' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for confidant that I've seen before include "Trusted friend" , "intimate" , "Sharer of secrets" , "Close friend entrusted with secrets etc" .)