His is a very exacting business (6)
I believe the answer is:
usurer
'his is a very exacting business' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for usurer that I've seen before include "Grasping money lender" , "Unrestrained money lender" , "Extortionist" , "One who charges excessive interest" , "Iniquitous moneylender" .)