That informative fellow at the bank? (6)
I believe the answer is:
teller
'bank?' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for teller that I've seen before include "one who counts" , "Bank official to narrate the story" , "Counter of votes or a bank employee" , "Bank official or reporter" , "Bank official or narrator" .)