A matter of extreme interest to the lender (5)
I believe the answer is:
usury
'lender' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for usury that I've seen before include "The lending of money at high rates" , "High-interest lending" , "Money-lending at very high rates" , "Practice of charging excessive interest" , "Lending money at excessive rates of interest" .)