Highly interesting business? (5)
I believe the answer is:
usury
'highly interesting business?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for usury that I've seen before include "High-rate money lending" , "Lending of money at exorbitant interest" , "The lending of money at high rates" , "Grasping moneylending" , "Taking a lot of interest" .)