Informant at the bank (6)
I believe the answer is:
teller
'informant' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for teller that I've seen before include "One records cash" , "Bank cashier or narrator of story" , "one reckons" , "one who counts" , "Election official" .)