Counter-informant (6)
I believe the answer is:
teller
'informant' is the definition.
(thesaurus)
I don't 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 "Bank cashier or narrator of story" , "Bank official or narrator" , "Edward ---, physicist; vote-counter" , "Counter of votes or a bank employee" , "Bank official to narrate the story" .)