Mark is employed in a bank and is guaranteed to work (4-4)
I believe the answer is:
fail-safe
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for fail-safe that I've seen before include "Accident-proof" , "It ensures a return to a safety when something goes wrong with a mechanism" , "Sure to succeed" , "Designed to prevent accidents in the case of malfunction" , "Unlikely to go wrong" .)