He reckons to work in a bank (6)
I believe the answer is:
teller
'he' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'reckons to work in a bank' is the wordplay.
'reckons' becomes 'er' (I can't justify this - if you can you should give a lot more credence to this answer).
'to' means one lot of letters go next to another (I've seen this in other clues).
'work in a bank' becomes 'tell' (I've seen this before).
'er' after 'tell' is 'TELLER'.
Can you help me to learn more?
(Other definitions for teller that I've seen before include "Edward ---, physicist; vote-counter" , "One who instructs" , "One introducing relation" , "one gives relation" , "One records cash" .)