If you want to know about banking, he’s your man! (6)
I believe the answer is:
teller
This might be a double definition.
'if you want to know about banking he's your' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'man' is the second definition.
The answer and definition can be both people as well as being singular nouns.
Maybe there's a link between them I don't understand?
Can you help me to learn more?
(Other definitions for teller that I've seen before include "One introducing relation" , "Counter of votes or a bank employee" , "Cashier - vote counter - narrator" , "one reckons" , "One records cash" .)